diff --git a/include/app/Knob.hpp b/include/app/Knob.hpp
index 5e2f3746..a219f0cf 100644
--- a/include/app/Knob.hpp
+++ b/include/app/Knob.hpp
@@ -15,10 +15,10 @@ struct Knob : ParamWidget {
 	/** Multiplier for mouse movement to adjust knob value */
 	float speed = 1.0;
 
-	void onButton(event::Button &e) override;
-	void onDragStart(event::DragStart &e) override;
-	void onDragEnd(event::DragEnd &e) override;
-	void onDragMove(event::DragMove &e) override;
+	void onButton(const event::Button &e) override;
+	void onDragStart(const event::DragStart &e) override;
+	void onDragEnd(const event::DragEnd &e) override;
+	void onDragMove(const event::DragMove &e) override;
 };
 
 
diff --git a/include/app/LedDisplay.hpp b/include/app/LedDisplay.hpp
index b97da325..34a341d2 100644
--- a/include/app/LedDisplay.hpp
+++ b/include/app/LedDisplay.hpp
@@ -24,7 +24,7 @@ struct LedDisplayChoice : TransparentWidget {
 	NVGcolor color;
 	LedDisplayChoice();
 	void draw(NVGcontext *vg) override;
-	void onButton(event::Button &e) override;
+	void onButton(const event::Button &e) override;
 };
 
 struct LedDisplayTextField : TextField {
diff --git a/include/app/ModuleWidget.hpp b/include/app/ModuleWidget.hpp
index aa8609f2..ceec426a 100644
--- a/include/app/ModuleWidget.hpp
+++ b/include/app/ModuleWidget.hpp
@@ -64,12 +64,12 @@ struct ModuleWidget : OpaqueWidget {
 	void drawShadow(NVGcontext *vg);
 
 	math::Vec dragPos;
-	void onHover(event::Hover &e) override;
-	void onButton(event::Button &e) override;
-	void onHoverKey(event::HoverKey &e) override;
-	void onDragStart(event::DragStart &e) override;
-	void onDragEnd(event::DragEnd &e) override;
-	void onDragMove(event::DragMove &e) override;
+	void onHover(const event::Hover &e) override;
+	void onButton(const event::Button &e) override;
+	void onHoverKey(const event::HoverKey &e) override;
+	void onDragStart(const event::DragStart &e) override;
+	void onDragEnd(const event::DragEnd &e) override;
+	void onDragMove(const event::DragMove &e) override;
 };
 
 
diff --git a/include/app/MomentarySwitch.hpp b/include/app/MomentarySwitch.hpp
index e2d14d3e..7ae69b40 100644
--- a/include/app/MomentarySwitch.hpp
+++ b/include/app/MomentarySwitch.hpp
@@ -10,8 +10,8 @@ namespace rack {
 Consider using SVGButton if the switch simply changes the state of your Module when clicked.
 */
 struct MomentarySwitch : virtual ParamWidget {
-	void onDragStart(event::DragStart &e) override;
-	void onDragEnd(event::DragEnd &e) override;
+	void onDragStart(const event::DragStart &e) override;
+	void onDragEnd(const event::DragEnd &e) override;
 };
 
 
diff --git a/include/app/ParamWidget.hpp b/include/app/ParamWidget.hpp
index 47b06c3d..789239a4 100644
--- a/include/app/ParamWidget.hpp
+++ b/include/app/ParamWidget.hpp
@@ -19,8 +19,8 @@ struct ParamWidget : OpaqueWidget {
 	void step() override;
 	/** For legacy patch loading */
 	void fromJson(json_t *rootJ);
-	void onButton(event::Button &e) override;
-	void onDragMove(event::DragMove &e) override;
+	void onButton(const event::Button &e) override;
+	void onDragMove(const event::DragMove &e) override;
 };
 
 
diff --git a/include/app/PortWidget.hpp b/include/app/PortWidget.hpp
index 498fff6d..bc0ed61d 100644
--- a/include/app/PortWidget.hpp
+++ b/include/app/PortWidget.hpp
@@ -23,12 +23,12 @@ struct PortWidget : OpaqueWidget {
 	~PortWidget();
 	void step() override;
 	void draw(NVGcontext *vg) override;
-	void onButton(event::Button &e) override;
-	void onDragStart(event::DragStart &e) override;
-	void onDragEnd(event::DragEnd &e) override;
-	void onDragDrop(event::DragDrop &e) override;
-	void onDragEnter(event::DragEnter &e) override;
-	void onDragLeave(event::DragLeave &e) override;
+	void onButton(const event::Button &e) override;
+	void onDragStart(const event::DragStart &e) override;
+	void onDragEnd(const event::DragEnd &e) override;
+	void onDragDrop(const event::DragDrop &e) override;
+	void onDragEnter(const event::DragEnter &e) override;
+	void onDragLeave(const event::DragLeave &e) override;
 };
 
 
diff --git a/include/app/RackWidget.hpp b/include/app/RackWidget.hpp
index fd42c628..2989a6b3 100644
--- a/include/app/RackWidget.hpp
+++ b/include/app/RackWidget.hpp
@@ -52,10 +52,10 @@ struct RackWidget : OpaqueWidget {
 	void step() override;
 	void draw(NVGcontext *vg) override;
 
-	void onHover(event::Hover &e) override;
-	void onDragHover(event::DragHover &e) override;
-	void onButton(event::Button &e) override;
-	void onZoom(event::Zoom &e) override;
+	void onHover(const event::Hover &e) override;
+	void onDragHover(const event::DragHover &e) override;
+	void onButton(const event::Button &e) override;
+	void onZoom(const event::Zoom &e) override;
 };
 
 
diff --git a/include/app/SVGButton.hpp b/include/app/SVGButton.hpp
index e27a7323..5f36ae5f 100644
--- a/include/app/SVGButton.hpp
+++ b/include/app/SVGButton.hpp
@@ -18,8 +18,8 @@ struct SVGButton : FramebufferWidget {
 	SVGButton();
 	/** If `activeSVG` is NULL, `defaultSVG` is used as the active state instead. */
 	void setSVGs(std::shared_ptr<SVG> defaultSVG, std::shared_ptr<SVG> activeSVG);
-	void onDragStart(event::DragStart &e) override;
-	void onDragEnd(event::DragEnd &e) override;
+	void onDragStart(const event::DragStart &e) override;
+	void onDragEnd(const event::DragEnd &e) override;
 };
 
 
diff --git a/include/app/SVGKnob.hpp b/include/app/SVGKnob.hpp
index 9d083fa4..117ff31e 100644
--- a/include/app/SVGKnob.hpp
+++ b/include/app/SVGKnob.hpp
@@ -21,7 +21,7 @@ struct SVGKnob : Knob, FramebufferWidget {
 	SVGKnob();
 	void setSVG(std::shared_ptr<SVG> svg);
 	void step() override;
-	void onChange(event::Change &e) override;
+	void onChange(const event::Change &e) override;
 };
 
 
diff --git a/include/app/SVGSlider.hpp b/include/app/SVGSlider.hpp
index 31631145..ebf916b2 100644
--- a/include/app/SVGSlider.hpp
+++ b/include/app/SVGSlider.hpp
@@ -20,7 +20,7 @@ struct SVGSlider : Knob, FramebufferWidget {
 	SVGSlider();
 	void setSVGs(std::shared_ptr<SVG> backgroundSVG, std::shared_ptr<SVG> handleSVG);
 	void step() override;
-	void onChange(event::Change &e) override;
+	void onChange(const event::Change &e) override;
 };
 
 
diff --git a/include/app/SVGSwitch.hpp b/include/app/SVGSwitch.hpp
index 8252b14f..a9b1d2b4 100644
--- a/include/app/SVGSwitch.hpp
+++ b/include/app/SVGSwitch.hpp
@@ -18,7 +18,7 @@ struct SVGSwitch : virtual ParamWidget, FramebufferWidget {
 	void step() override;
 	/** Adds an SVG file to represent the next switch position */
 	void addFrame(std::shared_ptr<SVG> svg);
-	void onChange(event::Change &e) override;
+	void onChange(const event::Change &e) override;
 };
 
 
diff --git a/include/app/Scene.hpp b/include/app/Scene.hpp
index 9f669f88..47597741 100644
--- a/include/app/Scene.hpp
+++ b/include/app/Scene.hpp
@@ -27,8 +27,8 @@ struct Scene : OpaqueWidget {
 	Scene();
 	void step() override;
 	void draw(NVGcontext *vg) override;
-	void onHoverKey(event::HoverKey &e) override;
-	void onPathDrop(event::PathDrop &e) override;
+	void onHoverKey(const event::HoverKey &e) override;
+	void onPathDrop(const event::PathDrop &e) override;
 
 	void runCheckVersion();
 };
diff --git a/include/app/ToggleSwitch.hpp b/include/app/ToggleSwitch.hpp
index 30642d99..51695a7e 100644
--- a/include/app/ToggleSwitch.hpp
+++ b/include/app/ToggleSwitch.hpp
@@ -8,7 +8,7 @@ namespace rack {
 
 /** A switch that cycles through each mechanical position */
 struct ToggleSwitch : virtual ParamWidget {
-	void onDragStart(event::DragStart &e) override;
+	void onDragStart(const event::DragStart &e) override;
 };
 
 
diff --git a/include/ui/Button.hpp b/include/ui/Button.hpp
index d9a93f28..d687fc02 100644
--- a/include/ui/Button.hpp
+++ b/include/ui/Button.hpp
@@ -26,27 +26,27 @@ struct Button : OpaqueWidget {
 		bndToolButton(vg, 0.0, 0.0, box.size.x, box.size.y, BND_CORNER_NONE, state, -1, text.c_str());
 	}
 
-	void onEnter(event::Enter &e) override {
+	void onEnter(const event::Enter &e) override {
 		state = BND_HOVER;
 	}
 
-	void onLeave(event::Leave &e) override {
+	void onLeave(const event::Leave &e) override {
 		state = BND_DEFAULT;
 	}
 
-	void onDragStart(event::DragStart &e) override {
+	void onDragStart(const event::DragStart &e) override {
 		state = BND_ACTIVE;
 		if (quantity)
 			quantity->setMax();
 	}
 
-	void onDragEnd(event::DragEnd &e) override {
+	void onDragEnd(const event::DragEnd &e) override {
 		state = BND_HOVER;
 		if (quantity)
 			quantity->setMin();
 	}
 
-	void onDragDrop(event::DragDrop &e) override {
+	void onDragDrop(const event::DragDrop &e) override {
 		if (e.origin == this) {
 			event::Action eAction;
 			onAction(eAction);
diff --git a/include/ui/Menu.hpp b/include/ui/Menu.hpp
index e046d32b..87b6e187 100644
--- a/include/ui/Menu.hpp
+++ b/include/ui/Menu.hpp
@@ -18,7 +18,7 @@ struct Menu : OpaqueWidget {
 	void setChildMenu(Menu *menu);
 	void step() override;
 	void draw(NVGcontext *vg) override;
-	void onHoverScroll(event::HoverScroll &e) override;
+	void onHoverScroll(const event::HoverScroll &e) override;
 };
 
 
diff --git a/include/ui/MenuItem.hpp b/include/ui/MenuItem.hpp
index 637a0d17..532b4bb3 100644
--- a/include/ui/MenuItem.hpp
+++ b/include/ui/MenuItem.hpp
@@ -19,8 +19,8 @@ struct MenuItem : MenuEntry {
 
 	void draw(NVGcontext *vg) override;
 	void step() override;
-	void onEnter(event::Enter &e) override;
-	void onDragDrop(event::DragDrop &e) override;
+	void onEnter(const event::Enter &e) override;
+	void onDragDrop(const event::DragDrop &e) override;
 	void doAction();
 	virtual Menu *createChildMenu() {return NULL;}
 };
diff --git a/include/ui/MenuOverlay.hpp b/include/ui/MenuOverlay.hpp
index c7daae79..2b529152 100644
--- a/include/ui/MenuOverlay.hpp
+++ b/include/ui/MenuOverlay.hpp
@@ -20,7 +20,7 @@ struct MenuOverlay : OpaqueWidget {
 		Widget::step();
 	}
 
-	void onButton(event::Button &e) override {
+	void onButton(const event::Button &e) override {
 		OpaqueWidget::onButton(e);
 
 		if (e.getConsumed() == this && e.action == GLFW_PRESS && e.button == GLFW_MOUSE_BUTTON_LEFT) {
@@ -28,7 +28,7 @@ struct MenuOverlay : OpaqueWidget {
 		}
 	}
 
-	void onHoverKey(event::HoverKey &e) override {
+	void onHoverKey(const event::HoverKey &e) override {
 		OpaqueWidget::onHoverKey(e);
 
 		if (e.getConsumed() == this && e.action == GLFW_PRESS && e.key == GLFW_KEY_ESCAPE) {
diff --git a/include/ui/RadioButton.hpp b/include/ui/RadioButton.hpp
index cf5c6ebd..2932a9b8 100644
--- a/include/ui/RadioButton.hpp
+++ b/include/ui/RadioButton.hpp
@@ -26,17 +26,17 @@ struct RadioButton : OpaqueWidget {
 		bndRadioButton(vg, 0.0, 0.0, box.size.x, box.size.y, BND_CORNER_NONE, state, -1, label.c_str());
 	}
 
-	void onEnter(event::Enter &e) override {
+	void onEnter(const event::Enter &e) override {
 		if (state != BND_ACTIVE)
 			state = BND_HOVER;
 	}
 
-	void onLeave(event::Leave &e) override {
+	void onLeave(const event::Leave &e) override {
 		if (state != BND_ACTIVE)
 			state = BND_DEFAULT;
 	}
 
-	void onDragDrop(event::DragDrop &e) override {
+	void onDragDrop(const event::DragDrop &e) override {
 		if (e.origin == this) {
 			if (state == BND_ACTIVE) {
 				state = BND_HOVER;
diff --git a/include/ui/ScrollWidget.hpp b/include/ui/ScrollWidget.hpp
index c1a05c5d..4db2fbfe 100644
--- a/include/ui/ScrollWidget.hpp
+++ b/include/ui/ScrollWidget.hpp
@@ -27,14 +27,14 @@ struct ScrollBar : OpaqueWidget {
 		bndScrollBar(vg, 0.0, 0.0, box.size.x, box.size.y, state, offset, size);
 	}
 
-	void onDragStart(event::DragStart &e) override {
+	void onDragStart(const event::DragStart &e) override {
 		state = BND_ACTIVE;
 		context()->window->cursorLock();
 	}
 
-	void onDragMove(event::DragMove &e) override;
+	void onDragMove(const event::DragMove &e) override;
 
-	void onDragEnd(event::DragEnd &e) override {
+	void onDragEnd(const event::DragEnd &e) override {
 		state = BND_DEFAULT;
 		context()->window->cursorUnlock();
 	}
@@ -110,7 +110,7 @@ struct ScrollWidget : OpaqueWidget {
 		verticalScrollBar->box.size.y = horizontalScrollBar->visible ? inner.y : box.size.y;
 	}
 
-	void onHover(event::Hover &e) override {
+	void onHover(const event::Hover &e) override {
 		// Scroll with arrow keys
 		if (!context()->event->selectedWidget) {
 			float arrowSpeed = 30.0;
@@ -138,14 +138,14 @@ struct ScrollWidget : OpaqueWidget {
 		OpaqueWidget::onHover(e);
 	}
 
-	void onHoverScroll(event::HoverScroll &e) override {
+	void onHoverScroll(const event::HoverScroll &e) override {
 		offset = offset.minus(e.scrollDelta);
 		e.consume(this);
 	}
 };
 
 
-inline void ScrollBar::onDragMove(event::DragMove &e) {
+inline void ScrollBar::onDragMove(const event::DragMove &e) {
 	ScrollWidget *scrollWidget = dynamic_cast<ScrollWidget*>(parent);
 	assert(scrollWidget);
 	if (orientation == HORIZONTAL)
diff --git a/include/ui/Slider.hpp b/include/ui/Slider.hpp
index 04448d44..50d923be 100644
--- a/include/ui/Slider.hpp
+++ b/include/ui/Slider.hpp
@@ -30,23 +30,23 @@ struct Slider : OpaqueWidget {
 		bndSlider(vg, 0.0, 0.0, box.size.x, box.size.y, BND_CORNER_NONE, state, progress, text.c_str(), NULL);
 	}
 
-	void onDragStart(event::DragStart &e) override {
+	void onDragStart(const event::DragStart &e) override {
 		state = BND_ACTIVE;
 		context()->window->cursorLock();
 	}
 
-	void onDragMove(event::DragMove &e) override {
+	void onDragMove(const event::DragMove &e) override {
 		if (quantity) {
 			quantity->moveScaledValue(SLIDER_SENSITIVITY * e.mouseDelta.x);
 		}
 	}
 
-	void onDragEnd(event::DragEnd &e) override {
+	void onDragEnd(const event::DragEnd &e) override {
 		state = BND_DEFAULT;
 		context()->window->cursorUnlock();
 	}
 
-	void onButton(event::Button &e) override {
+	void onButton(const event::Button &e) override {
 		if (e.action == GLFW_PRESS && e.button == GLFW_MOUSE_BUTTON_RIGHT) {
 			if (quantity)
 				quantity->reset();
diff --git a/include/ui/TextField.hpp b/include/ui/TextField.hpp
index 6abc2d1b..52d212b8 100644
--- a/include/ui/TextField.hpp
+++ b/include/ui/TextField.hpp
@@ -21,11 +21,11 @@ struct TextField : OpaqueWidget {
 
 	TextField();
 	void draw(NVGcontext *vg) override;
-	void onButton(event::Button &e) override;
-	void onHover(event::Hover &e) override;
-	void onEnter(event::Enter &e) override;
-	void onSelectText(event::SelectText &e) override;
-	void onSelectKey(event::SelectKey &e) override;
+	void onButton(const event::Button &e) override;
+	void onHover(const event::Hover &e) override;
+	void onEnter(const event::Enter &e) override;
+	void onSelectText(const event::SelectText &e) override;
+	void onSelectKey(const event::SelectKey &e) override;
 
 	/** Inserts text at the cursor, replacing the selection if necessary */
 	void insertText(std::string text);
diff --git a/include/ui/WindowWidget.hpp b/include/ui/WindowWidget.hpp
index e8b6befd..0ca43c45 100644
--- a/include/ui/WindowWidget.hpp
+++ b/include/ui/WindowWidget.hpp
@@ -14,7 +14,7 @@ struct WindowWidget : OpaqueWidget {
 		Widget::draw(vg);
 	}
 
-	void onDragMove(event::DragMove &e) override {
+	void onDragMove(const event::DragMove &e) override {
 		box.pos = box.pos.plus(e.mouseDelta);
 	}
 };
diff --git a/include/widgets/FramebufferWidget.hpp b/include/widgets/FramebufferWidget.hpp
index 4e32c0ed..c170739c 100644
--- a/include/widgets/FramebufferWidget.hpp
+++ b/include/widgets/FramebufferWidget.hpp
@@ -27,7 +27,7 @@ struct FramebufferWidget : virtual Widget {
 	void draw(NVGcontext *vg) override;
 	int getImageHandle();
 
-	void onZoom(event::Zoom &e) override {
+	void onZoom(const event::Zoom &e) override {
 		dirty = true;
 		Widget::onZoom(e);
 	}
diff --git a/include/widgets/OpaqueWidget.hpp b/include/widgets/OpaqueWidget.hpp
index 74a29712..14203c8f 100644
--- a/include/widgets/OpaqueWidget.hpp
+++ b/include/widgets/OpaqueWidget.hpp
@@ -10,37 +10,37 @@ You can of course override the events.
 You may also call OpaqueWidget::on*() from the overridden method to continue recursing/consuming the event.
 */
 struct OpaqueWidget : virtual Widget {
-	void onHover(event::Hover &e) override {
+	void onHover(const event::Hover &e) override {
 		Widget::onHover(e);
 		if (!e.getConsumed())
 			e.consume(this);
 	}
-	void onButton(event::Button &e) override {
+	void onButton(const event::Button &e) override {
 		Widget::onButton(e);
 		if (!e.getConsumed())
 			e.consume(this);
 	}
-	void onHoverKey(event::HoverKey &e) override {
+	void onHoverKey(const event::HoverKey &e) override {
 		Widget::onHoverKey(e);
 		if (!e.getConsumed())
 			e.consume(this);
 	}
-	void onHoverText(event::HoverText &e) override {
+	void onHoverText(const event::HoverText &e) override {
 		Widget::onHoverText(e);
 		if (!e.getConsumed())
 			e.consume(this);
 	}
-	void onHoverScroll(event::HoverScroll &e) override {
+	void onHoverScroll(const event::HoverScroll &e) override {
 		Widget::onHoverScroll(e);
 		if (!e.getConsumed())
 			e.consume(this);
 	}
-	void onDragHover(event::DragHover &e) override {
+	void onDragHover(const event::DragHover &e) override {
 		Widget::onDragHover(e);
 		if (!e.getConsumed())
 			e.consume(this);
 	}
-	void onPathDrop(event::PathDrop &e) override {
+	void onPathDrop(const event::PathDrop &e) override {
 		Widget::onPathDrop(e);
 		if (!e.getConsumed())
 			e.consume(this);
diff --git a/include/widgets/TransparentWidget.hpp b/include/widgets/TransparentWidget.hpp
index f53910a5..b522cb1b 100644
--- a/include/widgets/TransparentWidget.hpp
+++ b/include/widgets/TransparentWidget.hpp
@@ -8,13 +8,13 @@ namespace rack {
 /** Widget that does not respond to events and does not pass events to children */
 struct TransparentWidget : virtual Widget {
 	/** Override behavior to do nothing instead. */
-	void onHover(event::Hover &e) override {}
-	void onButton(event::Button &e) override {}
-	void onHoverKey(event::HoverKey &e) override {}
-	void onHoverText(event::HoverText &e) override {}
-	void onHoverScroll(event::HoverScroll &e) override {}
-	void onDragHover(event::DragHover &e) override {}
-	void onPathDrop(event::PathDrop &e) override {}
+	void onHover(const event::Hover &e) override {}
+	void onButton(const event::Button &e) override {}
+	void onHoverKey(const event::HoverKey &e) override {}
+	void onHoverText(const event::HoverText &e) override {}
+	void onHoverScroll(const event::HoverScroll &e) override {}
+	void onDragHover(const event::DragHover &e) override {}
+	void onPathDrop(const event::PathDrop &e) override {}
 };
 
 
diff --git a/include/widgets/Widget.hpp b/include/widgets/Widget.hpp
index 8d65c170..50c3dfd2 100644
--- a/include/widgets/Widget.hpp
+++ b/include/widgets/Widget.hpp
@@ -75,7 +75,7 @@ struct Widget {
 	// Events
 
 	template <typename TMethod, class TEvent>
-	void recurseEvent(TMethod f, TEvent &e) {
+	void recurseEvent(TMethod f, const TEvent &e) {
 		for (auto it = children.rbegin(); it != children.rend(); it++) {
 			Widget *child = *it;
 			// Filter child by visibility
@@ -91,7 +91,7 @@ struct Widget {
 	}
 
 	template <typename TMethod, class TEvent>
-	void recursePositionEvent(TMethod f, TEvent &e) {
+	void recursePositionEvent(TMethod f, const TEvent &e) {
 		for (auto it = children.rbegin(); it != children.rend(); it++) {
 			Widget *child = *it;
 			// Filter child by visibility and position
@@ -114,28 +114,28 @@ struct Widget {
 	/** Override these event callbacks to respond to events.
 	See events.hpp for a description of each event.
 	*/
-	virtual void onHover(event::Hover &e) {recursePositionEvent(&Widget::onHover, e);}
-	virtual void onButton(event::Button &e) {recursePositionEvent(&Widget::onButton, e);}
-	virtual void onHoverKey(event::HoverKey &e) {recursePositionEvent(&Widget::onHoverKey, e);}
-	virtual void onHoverText(event::HoverText &e) {recursePositionEvent(&Widget::onHoverText, e);}
-	virtual void onHoverScroll(event::HoverScroll &e) {recursePositionEvent(&Widget::onHoverScroll, e);}
-	virtual void onEnter(event::Enter &e) {}
-	virtual void onLeave(event::Leave &e) {}
-	virtual void onSelect(event::Select &e) {}
-	virtual void onDeselect(event::Deselect &e) {}
-	virtual void onSelectKey(event::SelectKey &e) {}
-	virtual void onSelectText(event::SelectText &e) {}
-	virtual void onDragStart(event::DragStart &e) {}
-	virtual void onDragEnd(event::DragEnd &e) {}
-	virtual void onDragMove(event::DragMove &e) {}
-	virtual void onDragHover(event::DragHover &e) {recursePositionEvent(&Widget::onDragHover, e);}
-	virtual void onDragEnter(event::DragEnter &e) {}
-	virtual void onDragLeave(event::DragLeave &e) {}
-	virtual void onDragDrop(event::DragDrop &e) {}
-	virtual void onPathDrop(event::PathDrop &e) {recursePositionEvent(&Widget::onPathDrop, e);}
-	virtual void onAction(event::Action &e) {}
-	virtual void onChange(event::Change &e) {}
-	virtual void onZoom(event::Zoom &e) {recurseEvent(&Widget::onZoom, e);}
+	virtual void onHover(const event::Hover &e) {recursePositionEvent(&Widget::onHover, e);}
+	virtual void onButton(const event::Button &e) {recursePositionEvent(&Widget::onButton, e);}
+	virtual void onHoverKey(const event::HoverKey &e) {recursePositionEvent(&Widget::onHoverKey, e);}
+	virtual void onHoverText(const event::HoverText &e) {recursePositionEvent(&Widget::onHoverText, e);}
+	virtual void onHoverScroll(const event::HoverScroll &e) {recursePositionEvent(&Widget::onHoverScroll, e);}
+	virtual void onEnter(const event::Enter &e) {}
+	virtual void onLeave(const event::Leave &e) {}
+	virtual void onSelect(const event::Select &e) {}
+	virtual void onDeselect(const event::Deselect &e) {}
+	virtual void onSelectKey(const event::SelectKey &e) {}
+	virtual void onSelectText(const event::SelectText &e) {}
+	virtual void onDragStart(const event::DragStart &e) {}
+	virtual void onDragEnd(const event::DragEnd &e) {}
+	virtual void onDragMove(const event::DragMove &e) {}
+	virtual void onDragHover(const event::DragHover &e) {recursePositionEvent(&Widget::onDragHover, e);}
+	virtual void onDragEnter(const event::DragEnter &e) {}
+	virtual void onDragLeave(const event::DragLeave &e) {}
+	virtual void onDragDrop(const event::DragDrop &e) {}
+	virtual void onPathDrop(const event::PathDrop &e) {recursePositionEvent(&Widget::onPathDrop, e);}
+	virtual void onAction(const event::Action &e) {}
+	virtual void onChange(const event::Change &e) {}
+	virtual void onZoom(const event::Zoom &e) {recurseEvent(&Widget::onZoom, e);}
 };
 
 
diff --git a/include/widgets/ZoomWidget.hpp b/include/widgets/ZoomWidget.hpp
index 8149023e..1b6184bc 100644
--- a/include/widgets/ZoomWidget.hpp
+++ b/include/widgets/ZoomWidget.hpp
@@ -35,33 +35,40 @@ struct ZoomWidget : virtual Widget {
 		Widget::draw(vg);
 	}
 
-	void onHover(event::Hover &e) override {
-		e.pos = e.pos.div(zoom);
-		Widget::onHover(e);
+	void onHover(const event::Hover &e) override {
+		event::Hover e2 = e;
+		e2.pos = e.pos.div(zoom);
+		Widget::onHover(e2);
 	}
-	void onButton(event::Button &e) override {
-		e.pos = e.pos.div(zoom);
-		Widget::onButton(e);
+	void onButton(const event::Button &e) override {
+		event::Button e2 = e;
+		e2.pos = e.pos.div(zoom);
+		Widget::onButton(e2);
 	}
-	void onHoverKey(event::HoverKey &e) override {
-		e.pos = e.pos.div(zoom);
-		Widget::onHoverKey(e);
+	void onHoverKey(const event::HoverKey &e) override {
+		event::HoverKey e2 = e;
+		e2.pos = e.pos.div(zoom);
+		Widget::onHoverKey(e2);
 	}
-	void onHoverText(event::HoverText &e) override {
-		e.pos = e.pos.div(zoom);
-		Widget::onHoverText(e);
+	void onHoverText(const event::HoverText &e) override {
+		event::HoverText e2 = e;
+		e2.pos = e.pos.div(zoom);
+		Widget::onHoverText(e2);
 	}
-	void onHoverScroll(event::HoverScroll &e) override {
-		e.pos = e.pos.div(zoom);
-		Widget::onHoverScroll(e);
+	void onHoverScroll(const event::HoverScroll &e) override {
+		event::HoverScroll e2 = e;
+		e2.pos = e.pos.div(zoom);
+		Widget::onHoverScroll(e2);
 	}
-	void onDragHover(event::DragHover &e) override {
-		e.pos = e.pos.div(zoom);
-		Widget::onDragHover(e);
+	void onDragHover(const event::DragHover &e) override {
+		event::DragHover e2 = e;
+		e2.pos = e.pos.div(zoom);
+		Widget::onDragHover(e2);
 	}
-	void onPathDrop(event::PathDrop &e) override {
-		e.pos = e.pos.div(zoom);
-		Widget::onPathDrop(e);
+	void onPathDrop(const event::PathDrop &e) override {
+		event::PathDrop e2 = e;
+		e2.pos = e.pos.div(zoom);
+		Widget::onPathDrop(e2);
 	}
 };
 
diff --git a/src/Core/Blank.cpp b/src/Core/Blank.cpp
index 7f57ed74..f143548e 100644
--- a/src/Core/Blank.cpp
+++ b/src/Core/Blank.cpp
@@ -11,18 +11,18 @@ struct ModuleResizeHandle : virtual Widget {
 	ModuleResizeHandle() {
 		box.size = Vec(RACK_GRID_WIDTH * 1, RACK_GRID_HEIGHT);
 	}
-	void onHover(event::Hover &e) override {
+	void onHover(const event::Hover &e) override {
 		// TODO
 		// if (e.button == 0) {
 		// 	e.target = this;
 		// }
 	}
-	void onDragStart(event::DragStart &e) override {
+	void onDragStart(const event::DragStart &e) override {
 		dragX = context()->scene->rackWidget->lastMousePos.x;
 		ModuleWidget *m = getAncestorOfType<ModuleWidget>();
 		originalBox = m->box;
 	}
-	void onDragMove(event::DragMove &e) override {
+	void onDragMove(const event::DragMove &e) override {
 		ModuleWidget *m = getAncestorOfType<ModuleWidget>();
 
 		float newDragX = context()->scene->rackWidget->lastMousePos.x;
diff --git a/src/Core/MIDICCToCVInterface.cpp b/src/Core/MIDICCToCVInterface.cpp
index fdab3a55..60802785 100644
--- a/src/Core/MIDICCToCVInterface.cpp
+++ b/src/Core/MIDICCToCVInterface.cpp
@@ -134,20 +134,20 @@ struct MidiCcChoice : GridChoice {
 		}
 	}
 
-	void onSelect(event::Select &e) override {
+	void onSelect(const event::Select &e) override {
 		e.consume(this);
 		module->learningId = id;
 		focusCc = -1;
 	}
 
-	void onDeselect(event::Deselect &e) override {
+	void onDeselect(const event::Deselect &e) override {
 		if (0 <= focusCc && focusCc < 128) {
 			module->learnedCcs[id] = focusCc;
 		}
 		module->learningId = -1;
 	}
 
-	void onSelectText(event::SelectText &e) override {
+	void onSelectText(const event::SelectText &e) override {
 		char c = e.codepoint;
 		if ('0' <= c && c <= '9') {
 			if (focusCc < 0)
@@ -157,7 +157,7 @@ struct MidiCcChoice : GridChoice {
 		e.consume(this);
 	}
 
-	void onSelectKey(event::SelectKey &e) override {
+	void onSelectKey(const event::SelectKey &e) override {
 		if (context()->event->selectedWidget == this) {
 			if (e.key == GLFW_KEY_ENTER || e.key == GLFW_KEY_KP_ENTER) {
 				event::Deselect eDeselect;
diff --git a/src/Core/MIDIToCVInterface.cpp b/src/Core/MIDIToCVInterface.cpp
index a7f2395d..2d712b2a 100644
--- a/src/Core/MIDIToCVInterface.cpp
+++ b/src/Core/MIDIToCVInterface.cpp
@@ -294,7 +294,7 @@ struct MIDIToCVInterfaceWidget : ModuleWidget {
 			MIDIToCVInterface *module;
 			int index;
 			int division;
-			void onAction(event::Action &e) override {
+			void onAction(const event::Action &e) override {
 				module->divisions[index] = division;
 			}
 		};
diff --git a/src/Core/MIDITriggerToCVInterface.cpp b/src/Core/MIDITriggerToCVInterface.cpp
index f206e35d..ee0bbc3c 100644
--- a/src/Core/MIDITriggerToCVInterface.cpp
+++ b/src/Core/MIDITriggerToCVInterface.cpp
@@ -178,12 +178,12 @@ struct MidiTrigChoice : GridChoice {
 		}
 	}
 
-	void onSelect(event::Select &e) override {
+	void onSelect(const event::Select &e) override {
 		e.consume(this);
 		module->learningId = id;
 	}
 
-	void onDeselect(event::Deselect &e) override {
+	void onDeselect(const event::Deselect &e) override {
 		module->learningId = -1;
 	}
 };
@@ -238,7 +238,7 @@ struct MIDITriggerToCVInterfaceWidget : ModuleWidget {
 
 		struct VelocityItem : MenuItem {
 			MIDITriggerToCVInterface *module;
-			void onAction(event::Action &e) override {
+			void onAction(const event::Action &e) override {
 				module->velocity ^= true;
 			}
 		};
diff --git a/src/Core/QuadMIDIToCVInterface.cpp b/src/Core/QuadMIDIToCVInterface.cpp
index e5905019..6aeaca21 100644
--- a/src/Core/QuadMIDIToCVInterface.cpp
+++ b/src/Core/QuadMIDIToCVInterface.cpp
@@ -339,7 +339,7 @@ struct QuadMIDIToCVInterfaceWidget : ModuleWidget {
 		struct PolyphonyItem : MenuItem {
 			QuadMIDIToCVInterface *module;
 			QuadMIDIToCVInterface::PolyMode polyMode;
-			void onAction(event::Action &e) override {
+			void onAction(const event::Action &e) override {
 				module->polyMode = polyMode;
 				module->onReset();
 			}
diff --git a/src/app/AudioWidget.cpp b/src/app/AudioWidget.cpp
index 880cc39c..712093ef 100644
--- a/src/app/AudioWidget.cpp
+++ b/src/app/AudioWidget.cpp
@@ -9,14 +9,14 @@ namespace rack {
 struct AudioDriverItem : MenuItem {
 	audio::IO *audioIO;
 	int driver;
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		audioIO->setDriver(driver);
 	}
 };
 
 struct AudioDriverChoice : LedDisplayChoice {
 	AudioWidget *audioWidget;
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		Menu *menu = createMenu();
 		menu->addChild(createMenuLabel("Audio driver"));
 		for (int driver : audioWidget->audioIO->getDrivers()) {
@@ -38,7 +38,7 @@ struct AudioDeviceItem : MenuItem {
 	audio::IO *audioIO;
 	int device;
 	int offset;
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		audioIO->setDevice(device, offset);
 	}
 };
@@ -48,7 +48,7 @@ struct AudioDeviceChoice : LedDisplayChoice {
 	/** Prevents devices with a ridiculous number of channels from being displayed */
 	int maxTotalChannels = 128;
 
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		Menu *menu = createMenu();
 		menu->addChild(createMenuLabel("Audio device"));
 		int deviceCount = audioWidget->audioIO->getDeviceCount();
@@ -89,14 +89,14 @@ struct AudioDeviceChoice : LedDisplayChoice {
 struct AudioSampleRateItem : MenuItem {
 	audio::IO *audioIO;
 	int sampleRate;
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		audioIO->setSampleRate(sampleRate);
 	}
 };
 
 struct AudioSampleRateChoice : LedDisplayChoice {
 	AudioWidget *audioWidget;
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		Menu *menu = createMenu();
 		menu->addChild(createMenuLabel("Sample rate"));
 		std::vector<int> sampleRates = audioWidget->audioIO->getSampleRates();
@@ -121,14 +121,14 @@ struct AudioSampleRateChoice : LedDisplayChoice {
 struct AudioBlockSizeItem : MenuItem {
 	audio::IO *audioIO;
 	int blockSize;
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		audioIO->setBlockSize(blockSize);
 	}
 };
 
 struct AudioBlockSizeChoice : LedDisplayChoice {
 	AudioWidget *audioWidget;
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		Menu *menu = createMenu();
 		menu->addChild(createMenuLabel("Block size"));
 		std::vector<int> blockSizes = audioWidget->audioIO->getBlockSizes();
diff --git a/src/app/Knob.cpp b/src/app/Knob.cpp
index 55ac1d61..01df7724 100644
--- a/src/app/Knob.cpp
+++ b/src/app/Knob.cpp
@@ -4,7 +4,7 @@
 namespace rack {
 
 
-void Knob::onButton(event::Button &e) {
+void Knob::onButton(const event::Button &e) {
 	float r = box.size.x / 2;
 	math::Vec c = box.size.div(2);
 	float dist = e.pos.minus(c).norm();
@@ -13,15 +13,15 @@ void Knob::onButton(event::Button &e) {
 	}
 }
 
-void Knob::onDragStart(event::DragStart &e) {
+void Knob::onDragStart(const event::DragStart &e) {
 	context()->window->cursorLock();
 }
 
-void Knob::onDragEnd(event::DragEnd &e) {
+void Knob::onDragEnd(const event::DragEnd &e) {
 	context()->window->cursorUnlock();
 }
 
-void Knob::onDragMove(event::DragMove &e) {
+void Knob::onDragMove(const event::DragMove &e) {
 	if (quantity) {
 		float range;
 		if (quantity->isBounded()) {
diff --git a/src/app/LedDisplay.cpp b/src/app/LedDisplay.cpp
index cfdfc6a7..62754095 100644
--- a/src/app/LedDisplay.cpp
+++ b/src/app/LedDisplay.cpp
@@ -54,7 +54,7 @@ void LedDisplayChoice::draw(NVGcontext *vg) {
 	nvgResetScissor(vg);
 }
 
-void LedDisplayChoice::onButton(event::Button &e) {
+void LedDisplayChoice::onButton(const event::Button &e) {
 	if (e.action == GLFW_PRESS && (e.button == GLFW_MOUSE_BUTTON_LEFT || e.button == GLFW_MOUSE_BUTTON_RIGHT)) {
 		event::Action eAction;
 		onAction(eAction);
diff --git a/src/app/MidiWidget.cpp b/src/app/MidiWidget.cpp
index de207811..41e8d44b 100644
--- a/src/app/MidiWidget.cpp
+++ b/src/app/MidiWidget.cpp
@@ -9,14 +9,14 @@ namespace rack {
 struct MidiDriverItem : MenuItem {
 	midi::IO *midiIO;
 	int driverId;
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		midiIO->setDriverId(driverId);
 	}
 };
 
 struct MidiDriverChoice : LedDisplayChoice {
 	MidiWidget *midiWidget;
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		Menu *menu = createMenu();
 		menu->addChild(createMenuLabel("MIDI driver"));
 		for (int driverId : midiWidget->midiIO->getDriverIds()) {
@@ -43,14 +43,14 @@ struct MidiDriverChoice : LedDisplayChoice {
 struct MidiDeviceItem : MenuItem {
 	midi::IO *midiIO;
 	int deviceId;
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		midiIO->setDeviceId(deviceId);
 	}
 };
 
 struct MidiDeviceChoice : LedDisplayChoice {
 	MidiWidget *midiWidget;
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		Menu *menu = createMenu();
 		menu->addChild(createMenuLabel("MIDI device"));
 		{
@@ -85,14 +85,14 @@ struct MidiDeviceChoice : LedDisplayChoice {
 struct MidiChannelItem : MenuItem {
 	midi::IO *midiIO;
 	int channel;
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		midiIO->channel = channel;
 	}
 };
 
 struct MidiChannelChoice : LedDisplayChoice {
 	MidiWidget *midiWidget;
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		Menu *menu = createMenu();
 		menu->addChild(createMenuLabel("MIDI channel"));
 		for (int channel = -1; channel < 16; channel++) {
diff --git a/src/app/ModuleBrowser.cpp b/src/app/ModuleBrowser.cpp
index efc3fcf4..c42a14c0 100644
--- a/src/app/ModuleBrowser.cpp
+++ b/src/app/ModuleBrowser.cpp
@@ -68,7 +68,7 @@ struct FavoriteRadioButton : RadioButton {
 		quantity = new FavoriteQuantity;
 	}
 
-	void onAction(event::Action &e) override;
+	void onAction(const event::Action &e) override;
 };
 
 
@@ -101,9 +101,9 @@ struct BrowserListItem : OpaqueWidget {
 		Widget::draw(vg);
 	}
 
-	void onDragStart(event::DragStart &e) override;
+	void onDragStart(const event::DragStart &e) override;
 
-	void onDragDrop(event::DragDrop &e) override {
+	void onDragDrop(const event::DragDrop &e) override {
 		if (e.origin != this)
 			return;
 		doAction();
@@ -159,7 +159,7 @@ struct ModelItem : BrowserListItem {
 			pluginLabel->box.size.x = box.size.x - BND_SCROLLBAR_WIDTH;
 	}
 
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		ModuleWidget *moduleWidget = model->createModuleWidget();
 		if (!moduleWidget)
 			return;
@@ -185,7 +185,7 @@ struct AuthorItem : BrowserListItem {
 		addChild(authorLabel);
 	}
 
-	void onAction(event::Action &e) override;
+	void onAction(const event::Action &e) override;
 };
 
 
@@ -203,7 +203,7 @@ struct TagItem : BrowserListItem {
 		addChild(tagLabel);
 	}
 
-	void onAction(event::Action &e) override;
+	void onAction(const event::Action &e) override;
 };
 
 
@@ -214,7 +214,7 @@ struct ClearFilterItem : BrowserListItem {
 		addChild(label);
 	}
 
-	void onAction(event::Action &e) override;
+	void onAction(const event::Action &e) override;
 };
 
 
@@ -294,8 +294,8 @@ struct ModuleBrowser;
 
 struct SearchModuleField : TextField {
 	ModuleBrowser *moduleBrowser;
-	void onChange(event::Change &e) override;
-	void onSelectKey(event::SelectKey &e) override;
+	void onChange(const event::Change &e) override;
+	void onSelectKey(const event::SelectKey &e) override;
 };
 
 
@@ -459,7 +459,7 @@ struct ModuleBrowser : OpaqueWidget {
 
 // Implementations of inline methods above
 
-void AuthorItem::onAction(event::Action &e) {
+void AuthorItem::onAction(const event::Action &e) {
 	ModuleBrowser *moduleBrowser = getAncestorOfType<ModuleBrowser>();
 	sAuthorFilter = author;
 	moduleBrowser->clearSearch();
@@ -467,7 +467,7 @@ void AuthorItem::onAction(event::Action &e) {
 	e.consume(this);
 }
 
-void TagItem::onAction(event::Action &e) {
+void TagItem::onAction(const event::Action &e) {
 	ModuleBrowser *moduleBrowser = getAncestorOfType<ModuleBrowser>();
 	sTagFilter = tag;
 	moduleBrowser->clearSearch();
@@ -475,7 +475,7 @@ void TagItem::onAction(event::Action &e) {
 	e.consume(this);
 }
 
-void ClearFilterItem::onAction(event::Action &e) {
+void ClearFilterItem::onAction(const event::Action &e) {
 	ModuleBrowser *moduleBrowser = getAncestorOfType<ModuleBrowser>();
 	sAuthorFilter = "";
 	sTagFilter = "";
@@ -483,7 +483,7 @@ void ClearFilterItem::onAction(event::Action &e) {
 	e.consume(this);
 }
 
-void FavoriteRadioButton::onAction(event::Action &e) {
+void FavoriteRadioButton::onAction(const event::Action &e) {
 	if (!model)
 		return;
 	if (quantity->isMax()) {
@@ -500,18 +500,18 @@ void FavoriteRadioButton::onAction(event::Action &e) {
 		moduleBrowser->refreshSearch();
 }
 
-void BrowserListItem::onDragStart(event::DragStart &e) {
+void BrowserListItem::onDragStart(const event::DragStart &e) {
 	BrowserList *list = dynamic_cast<BrowserList*>(parent);
 	if (list) {
 		list->selectItem(this);
 	}
 }
 
-void SearchModuleField::onChange(event::Change &e) {
+void SearchModuleField::onChange(const event::Change &e) {
 	moduleBrowser->refreshSearch();
 }
 
-void SearchModuleField::onSelectKey(event::SelectKey &e) {
+void SearchModuleField::onSelectKey(const event::SelectKey &e) {
 	if (e.action == GLFW_PRESS) {
 		switch (e.key) {
 			case GLFW_KEY_ESCAPE: {
diff --git a/src/app/ModuleWidget.cpp b/src/app/ModuleWidget.cpp
index 511b5a27..4f8a8819 100644
--- a/src/app/ModuleWidget.cpp
+++ b/src/app/ModuleWidget.cpp
@@ -315,7 +315,7 @@ void ModuleWidget::drawShadow(NVGcontext *vg) {
 	nvgFill(vg);
 }
 
-void ModuleWidget::onHover(event::Hover &e) {
+void ModuleWidget::onHover(const event::Hover &e) {
 	OpaqueWidget::onHover(e);
 
 	// Instead of checking key-down events, delete the module even if key-repeat hasn't fired yet and the cursor is hovering over the widget.
@@ -327,7 +327,7 @@ void ModuleWidget::onHover(event::Hover &e) {
 	}
 }
 
-void ModuleWidget::onButton(event::Button &e) {
+void ModuleWidget::onButton(const event::Button &e) {
 	OpaqueWidget::onButton(e);
 	if (e.getConsumed() == this) {
 		if (e.button == 1) {
@@ -336,7 +336,7 @@ void ModuleWidget::onButton(event::Button &e) {
 	}
 }
 
-void ModuleWidget::onHoverKey(event::HoverKey &e) {
+void ModuleWidget::onHoverKey(const event::HoverKey &e) {
 	if (e.action == GLFW_PRESS || e.action == GLFW_REPEAT) {
 		switch (e.key) {
 			case GLFW_KEY_I: {
@@ -382,14 +382,14 @@ void ModuleWidget::onHoverKey(event::HoverKey &e) {
 		OpaqueWidget::onHoverKey(e);
 }
 
-void ModuleWidget::onDragStart(event::DragStart &e) {
+void ModuleWidget::onDragStart(const event::DragStart &e) {
 	dragPos = context()->scene->rackWidget->lastMousePos.minus(box.pos);
 }
 
-void ModuleWidget::onDragEnd(event::DragEnd &e) {
+void ModuleWidget::onDragEnd(const event::DragEnd &e) {
 }
 
-void ModuleWidget::onDragMove(event::DragMove &e) {
+void ModuleWidget::onDragMove(const event::DragMove &e) {
 	if (!settings::lockModules) {
 		math::Rect newBox = box;
 		newBox.pos = context()->scene->rackWidget->lastMousePos.minus(dragPos);
@@ -404,7 +404,7 @@ struct ModuleDisconnectItem : MenuItem {
 		text = "Disconnect cables";
 		rightText = WINDOW_MOD_KEY_NAME "+U";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		moduleWidget->disconnect();
 	}
 };
@@ -415,7 +415,7 @@ struct ModuleResetItem : MenuItem {
 		text = "Initialize";
 		rightText = WINDOW_MOD_KEY_NAME "+I";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		moduleWidget->reset();
 	}
 };
@@ -426,7 +426,7 @@ struct ModuleRandomizeItem : MenuItem {
 		text = "Randomize";
 		rightText = WINDOW_MOD_KEY_NAME "+R";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		moduleWidget->randomize();
 	}
 };
@@ -437,7 +437,7 @@ struct ModuleCopyItem : MenuItem {
 		text = "Copy preset";
 		rightText = WINDOW_MOD_KEY_NAME "+C";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		moduleWidget->copyClipboard();
 	}
 };
@@ -448,7 +448,7 @@ struct ModulePasteItem : MenuItem {
 		text = "Paste preset";
 		rightText = WINDOW_MOD_KEY_NAME "+V";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		moduleWidget->pasteClipboard();
 	}
 };
@@ -458,7 +458,7 @@ struct ModuleSaveItem : MenuItem {
 	ModuleSaveItem() {
 		text = "Save preset";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		moduleWidget->saveDialog();
 	}
 };
@@ -468,7 +468,7 @@ struct ModuleLoadItem : MenuItem {
 	ModuleLoadItem() {
 		text = "Load preset";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		moduleWidget->loadDialog();
 	}
 };
@@ -479,7 +479,7 @@ struct ModuleCloneItem : MenuItem {
 		text = "Duplicate";
 		rightText = WINDOW_MOD_KEY_NAME "+D";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		context()->scene->rackWidget->cloneModule(moduleWidget);
 	}
 };
@@ -490,7 +490,7 @@ struct ModuleDeleteItem : MenuItem {
 		text = "Delete";
 		rightText = "Backspace/Delete";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		context()->scene->rackWidget->deleteModule(moduleWidget);
 		delete moduleWidget;
 	}
diff --git a/src/app/MomentarySwitch.cpp b/src/app/MomentarySwitch.cpp
index ff3d7e51..7e842903 100644
--- a/src/app/MomentarySwitch.cpp
+++ b/src/app/MomentarySwitch.cpp
@@ -4,13 +4,13 @@
 namespace rack {
 
 
-void MomentarySwitch::onDragStart(event::DragStart &e) {
+void MomentarySwitch::onDragStart(const event::DragStart &e) {
 	if (quantity) {
 		quantity->setMax();
 	}
 }
 
-void MomentarySwitch::onDragEnd(event::DragEnd &e) {
+void MomentarySwitch::onDragEnd(const event::DragEnd &e) {
 	if (quantity) {
 		quantity->setMin();
 	}
diff --git a/src/app/ParamWidget.cpp b/src/app/ParamWidget.cpp
index 85749983..14ea2a42 100644
--- a/src/app/ParamWidget.cpp
+++ b/src/app/ParamWidget.cpp
@@ -27,7 +27,7 @@ void ParamWidget::fromJson(json_t *rootJ) {
 	}
 }
 
-void ParamWidget::onButton(event::Button &e) {
+void ParamWidget::onButton(const event::Button &e) {
 	// Right click to reset
 	if (e.action == GLFW_PRESS && e.button == GLFW_MOUSE_BUTTON_RIGHT) {
 		if (quantity)
@@ -39,7 +39,7 @@ void ParamWidget::onButton(event::Button &e) {
 	OpaqueWidget::onButton(e);
 }
 
-void ParamWidget::onDragMove(event::DragMove &e) {
+void ParamWidget::onDragMove(const event::DragMove &e) {
 	if (quantity) {
 		DEBUG("%s", quantity->getString().c_str());
 	}
diff --git a/src/app/PortWidget.cpp b/src/app/PortWidget.cpp
index 44362627..a2e879e6 100644
--- a/src/app/PortWidget.cpp
+++ b/src/app/PortWidget.cpp
@@ -50,7 +50,7 @@ void PortWidget::draw(NVGcontext *vg) {
 	}
 }
 
-void PortWidget::onButton(event::Button &e) {
+void PortWidget::onButton(const event::Button &e) {
 	if (e.action == GLFW_PRESS && e.button == GLFW_MOUSE_BUTTON_RIGHT) {
 		context()->scene->rackWidget->wireContainer->removeTopWire(this);
 
@@ -62,7 +62,7 @@ void PortWidget::onButton(event::Button &e) {
 	e.consume(this);
 }
 
-void PortWidget::onDragStart(event::DragStart &e) {
+void PortWidget::onDragStart(const event::DragStart &e) {
 	// Try to grab wire on top of stack
 	WireWidget *wire = NULL;
 	if (type == INPUT || !context()->window->isModPressed()) {
@@ -82,13 +82,13 @@ void PortWidget::onDragStart(event::DragStart &e) {
 	context()->scene->rackWidget->wireContainer->setActiveWire(wire);
 }
 
-void PortWidget::onDragEnd(event::DragEnd &e) {
+void PortWidget::onDragEnd(const event::DragEnd &e) {
 	// FIXME
 	// If the source PortWidget is deleted, this will be called, removing the cable
 	context()->scene->rackWidget->wireContainer->commitActiveWire();
 }
 
-void PortWidget::onDragDrop(event::DragDrop &e) {
+void PortWidget::onDragDrop(const event::DragDrop &e) {
 	PortWidget *originPort = dynamic_cast<PortWidget*>(e.origin);
 	if (!originPort)
 		return;
@@ -99,7 +99,7 @@ void PortWidget::onDragDrop(event::DragDrop &e) {
 	onDragEnter(eDragEnter);
 }
 
-void PortWidget::onDragEnter(event::DragEnter &e) {
+void PortWidget::onDragEnter(const event::DragEnter &e) {
 	PortWidget *originPort = dynamic_cast<PortWidget*>(e.origin);
 	if (!originPort)
 		return;
@@ -117,7 +117,7 @@ void PortWidget::onDragEnter(event::DragEnter &e) {
 	}
 }
 
-void PortWidget::onDragLeave(event::DragLeave &e) {
+void PortWidget::onDragLeave(const event::DragLeave &e) {
 	PortWidget *originPort = dynamic_cast<PortWidget*>(e.origin);
 	if (!originPort)
 		return;
diff --git a/src/app/RackWidget.cpp b/src/app/RackWidget.cpp
index 1db99fb1..831a706d 100644
--- a/src/app/RackWidget.cpp
+++ b/src/app/RackWidget.cpp
@@ -539,17 +539,17 @@ void RackWidget::draw(NVGcontext *vg) {
 	Widget::draw(vg);
 }
 
-void RackWidget::onHover(event::Hover &e) {
+void RackWidget::onHover(const event::Hover &e) {
 	OpaqueWidget::onHover(e);
 	lastMousePos = e.pos;
 }
 
-void RackWidget::onDragHover(event::DragHover &e) {
+void RackWidget::onDragHover(const event::DragHover &e) {
 	OpaqueWidget::onDragHover(e);
 	lastMousePos = e.pos;
 }
 
-void RackWidget::onButton(event::Button &e) {
+void RackWidget::onButton(const event::Button &e) {
 	OpaqueWidget::onButton(e);
 	if (e.getConsumed() == this) {
 		if (e.action == GLFW_PRESS && e.button == GLFW_MOUSE_BUTTON_RIGHT) {
@@ -558,7 +558,7 @@ void RackWidget::onButton(event::Button &e) {
 	}
 }
 
-void RackWidget::onZoom(event::Zoom &e) {
+void RackWidget::onZoom(const event::Zoom &e) {
 	rails->box.size = math::Vec();
 	OpaqueWidget::onZoom(e);
 }
diff --git a/src/app/SVGButton.cpp b/src/app/SVGButton.cpp
index ea0eb937..fe13fc6b 100644
--- a/src/app/SVGButton.cpp
+++ b/src/app/SVGButton.cpp
@@ -16,14 +16,14 @@ void SVGButton::setSVGs(std::shared_ptr<SVG> defaultSVG, std::shared_ptr<SVG> ac
 	this->activeSVG = activeSVG ? activeSVG : defaultSVG;
 }
 
-void SVGButton::onDragStart(event::DragStart &e) {
+void SVGButton::onDragStart(const event::DragStart &e) {
 	event::Action eAction;
 	onAction(eAction);
 	sw->setSVG(activeSVG);
 	dirty = true;
 }
 
-void SVGButton::onDragEnd(event::DragEnd &e) {
+void SVGButton::onDragEnd(const event::DragEnd &e) {
 	sw->setSVG(defaultSVG);
 	dirty = true;
 }
diff --git a/src/app/SVGKnob.cpp b/src/app/SVGKnob.cpp
index cd63f951..2a4bdfb3 100644
--- a/src/app/SVGKnob.cpp
+++ b/src/app/SVGKnob.cpp
@@ -30,7 +30,7 @@ void SVGKnob::step() {
 	FramebufferWidget::step();
 }
 
-void SVGKnob::onChange(event::Change &e) {
+void SVGKnob::onChange(const event::Change &e) {
 	// Re-transform the TransformWidget
 	if (quantity) {
 		float angle;
diff --git a/src/app/SVGSlider.cpp b/src/app/SVGSlider.cpp
index e135b319..9595b6b7 100644
--- a/src/app/SVGSlider.cpp
+++ b/src/app/SVGSlider.cpp
@@ -27,7 +27,7 @@ void SVGSlider::step() {
 	FramebufferWidget::step();
 }
 
-void SVGSlider::onChange(event::Change &e) {
+void SVGSlider::onChange(const event::Change &e) {
 	if (quantity) {
 		// Interpolate handle position
 		float v = quantity->getScaledValue();
diff --git a/src/app/SVGSwitch.cpp b/src/app/SVGSwitch.cpp
index 549bca95..4e716593 100644
--- a/src/app/SVGSwitch.cpp
+++ b/src/app/SVGSwitch.cpp
@@ -23,7 +23,7 @@ void SVGSwitch::addFrame(std::shared_ptr<SVG> svg) {
 	}
 }
 
-void SVGSwitch::onChange(event::Change &e) {
+void SVGSwitch::onChange(const event::Change &e) {
 	assert(frames.size() > 0);
 	if (quantity) {
 		int index = quantity->getScaledValue() * (frames.size() - 1);
diff --git a/src/app/Scene.cpp b/src/app/Scene.cpp
index 0eeb5b48..bc78e63d 100644
--- a/src/app/Scene.cpp
+++ b/src/app/Scene.cpp
@@ -66,7 +66,7 @@ void Scene::draw(NVGcontext *vg) {
 	OpaqueWidget::draw(vg);
 }
 
-void Scene::onHoverKey(event::HoverKey &e) {
+void Scene::onHoverKey(const event::HoverKey &e) {
 	if (e.action == GLFW_PRESS) {
 		switch (e.key) {
 			case GLFW_KEY_N: {
@@ -123,7 +123,7 @@ void Scene::onHoverKey(event::HoverKey &e) {
 		OpaqueWidget::onHoverKey(e);
 }
 
-void Scene::onPathDrop(event::PathDrop &e) {
+void Scene::onPathDrop(const event::PathDrop &e) {
 	if (e.paths.size() >= 1) {
 		const std::string &path = e.paths[0];
 		if (string::extension(path) == "vcv") {
diff --git a/src/app/ToggleSwitch.cpp b/src/app/ToggleSwitch.cpp
index c0f2d38a..e0d2cc02 100644
--- a/src/app/ToggleSwitch.cpp
+++ b/src/app/ToggleSwitch.cpp
@@ -4,7 +4,7 @@
 namespace rack {
 
 
-void ToggleSwitch::onDragStart(event::DragStart &e) {
+void ToggleSwitch::onDragStart(const event::DragStart &e) {
 	// Cycle through values
 	// e.g. a range of [0.0, 3.0] would have modes 0, 1, 2, and 3.
 	if (quantity) {
diff --git a/src/app/Toolbar.cpp b/src/app/Toolbar.cpp
index 579847b6..f0942bc0 100644
--- a/src/app/Toolbar.cpp
+++ b/src/app/Toolbar.cpp
@@ -37,7 +37,7 @@ struct NewItem : MenuItem {
 		text = "New";
 		rightText = "(" WINDOW_MOD_KEY_NAME "+N)";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		context()->scene->rackWidget->reset();
 	}
 };
@@ -48,7 +48,7 @@ struct OpenItem : MenuItem {
 		text = "Open";
 		rightText = "(" WINDOW_MOD_KEY_NAME "+O)";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		context()->scene->rackWidget->loadDialog();
 	}
 };
@@ -59,7 +59,7 @@ struct SaveItem : MenuItem {
 		text = "Save";
 		rightText = "(" WINDOW_MOD_KEY_NAME "+S)";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		context()->scene->rackWidget->saveDialog();
 	}
 };
@@ -70,7 +70,7 @@ struct SaveAsItem : MenuItem {
 		text = "Save as";
 		rightText = "(" WINDOW_MOD_KEY_NAME "+Shift+S)";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		context()->scene->rackWidget->saveAsDialog();
 	}
 };
@@ -80,7 +80,7 @@ struct RevertItem : MenuItem {
 	RevertItem() {
 		text = "Revert";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		context()->scene->rackWidget->revert();
 	}
 };
@@ -90,7 +90,7 @@ struct DisconnectCablesItem : MenuItem {
 	DisconnectCablesItem() {
 		text = "Disconnect cables";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		context()->scene->rackWidget->disconnect();
 	}
 };
@@ -100,7 +100,7 @@ struct FileButton : MenuButton {
 	FileButton() {
 		text = "File";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		Menu *menu = createMenu();
 		menu->box.pos = getAbsoluteOffset(math::Vec(0, box.size.y));
 		menu->box.size.x = box.size.x;
@@ -168,7 +168,7 @@ struct PowerMeterItem : MenuItem {
 		text = "Power meter";
 		rightText = CHECKMARK(settings::powerMeter);
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		settings::powerMeter ^= true;
 	}
 };
@@ -179,7 +179,7 @@ struct LockModulesItem : MenuItem {
 		text = "Lock modules";
 		rightText = CHECKMARK(settings::lockModules);
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		settings::lockModules ^= true;
 	}
 };
@@ -190,7 +190,7 @@ struct EnginePauseItem : MenuItem {
 		text = "Pause engine";
 		rightText = CHECKMARK(context()->engine->paused);
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		context()->engine->paused ^= true;
 	}
 };
@@ -203,7 +203,7 @@ struct SampleRateValueItem : MenuItem {
 		text = string::f("%.0f Hz", sampleRate);
 		rightText = CHECKMARK(context()->engine->getSampleRate() == sampleRate);
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		context()->engine->setSampleRate(sampleRate);
 		context()->engine->paused = false;
 	}
@@ -232,7 +232,7 @@ struct SettingsButton : MenuButton {
 	SettingsButton() {
 		text = "Settings";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		Menu *menu = createMenu();
 		menu->box.pos = getAbsoluteOffset(math::Vec(0, box.size.y));
 		menu->box.size.x = box.size.x;
@@ -263,7 +263,7 @@ struct RegisterItem : MenuItem {
 	RegisterItem() {
 		text = "Register VCV account";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		std::thread t([&]() {
 			system::openBrowser("https://vcvrack.com/");
 		});
@@ -277,7 +277,7 @@ struct AccountEmailField : TextField {
 	AccountEmailField() {
 		placeholder = "Email";
 	}
-	void onSelectKey(event::SelectKey &e) override {
+	void onSelectKey(const event::SelectKey &e) override {
 		if (e.action == GLFW_PRESS && e.key == GLFW_KEY_TAB) {
 			context()->event->selectedWidget = passwordField;
 			e.consume(this);
@@ -293,7 +293,7 @@ struct AccountPasswordField : PasswordField {
 	AccountPasswordField() {
 		placeholder = "Password";
 	}
-	void onSelectKey(event::SelectKey &e) override {
+	void onSelectKey(const event::SelectKey &e) override {
 		if (e.action == GLFW_PRESS && (e.key == GLFW_KEY_ENTER || e.key == GLFW_KEY_KP_ENTER)) {
 			logInItem->doAction();
 			e.consume(this);
@@ -310,7 +310,7 @@ struct LogInItem : MenuItem {
 	LogInItem() {
 		text = "Log in";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		std::string email = emailField->text;
 		std::string password = passwordField->text;
 		std::thread t([&, email, password]() {
@@ -325,7 +325,7 @@ struct ManageItem : MenuItem {
 	ManageItem() {
 		text = "Manage plugins";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		std::thread t([&]() {
 			system::openBrowser("https://vcvrack.com/plugins.html");
 		});
@@ -339,7 +339,7 @@ struct SyncItem : MenuItem {
 		text = "Sync plugins";
 		disabled = true;
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 	}
 };
 
@@ -369,7 +369,7 @@ struct SyncItem : MenuItem {
 // 			completed = false;
 // 		}
 // 	}
-// 	void onAction(event::Action &e) override {
+// 	void onAction(const event::Action &e) override {
 // 		available = false;
 // 		std::thread t([this]() {
 // 			if (plugin::sync(false))
@@ -384,7 +384,7 @@ struct LogOutItem : MenuItem {
 	LogOutItem() {
 		text = "Log out";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		plugin::logOut();
 	}
 };
@@ -413,7 +413,7 @@ struct PluginsButton : MenuButton {
 	PluginsButton() {
 		text = "Plugins";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		Menu *menu = createMenu();
 		menu->box.pos = getAbsoluteOffset(math::Vec(0, box.size.y));
 		menu->box.size.x = box.size.x;
@@ -465,7 +465,7 @@ struct ManualItem : MenuItem {
 	ManualItem() {
 		text = "Manual";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		std::thread t([&]() {
 			system::openBrowser("https://vcvrack.com/manual/");
 		});
@@ -478,7 +478,7 @@ struct WebsiteItem : MenuItem {
 	WebsiteItem() {
 		text = "VCVRack.com";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		std::thread t([&]() {
 			system::openBrowser("https://vcvrack.com/");
 		});
@@ -492,7 +492,7 @@ struct CheckVersionItem : MenuItem {
 		text = "Check version on launch";
 		rightText = CHECKMARK(settings::checkVersion);
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		settings::checkVersion ^= true;
 	}
 };
@@ -502,7 +502,7 @@ struct HelpButton : MenuButton {
 	HelpButton() {
 		text = "Help";
 	}
-	void onAction(event::Action &e) override {
+	void onAction(const event::Action &e) override {
 		Menu *menu = createMenu();
 		menu->box.pos = getAbsoluteOffset(math::Vec(0, box.size.y));
 		menu->box.size.x = box.size.x;
diff --git a/src/ui/Menu.cpp b/src/ui/Menu.cpp
index 3be22bbd..febff676 100644
--- a/src/ui/Menu.cpp
+++ b/src/ui/Menu.cpp
@@ -53,7 +53,7 @@ void Menu::draw(NVGcontext *vg) {
 	Widget::draw(vg);
 }
 
-void Menu::onHoverScroll(event::HoverScroll &e) {
+void Menu::onHoverScroll(const event::HoverScroll &e) {
 	if (parent && !parent->box.contains(box))
 		box.pos.y += e.scrollDelta.y;
 }
diff --git a/src/ui/MenuItem.cpp b/src/ui/MenuItem.cpp
index a7e6063e..55f188a6 100644
--- a/src/ui/MenuItem.cpp
+++ b/src/ui/MenuItem.cpp
@@ -35,7 +35,7 @@ void MenuItem::step() {
 	Widget::step();
 }
 
-void MenuItem::onEnter(event::Enter &e) {
+void MenuItem::onEnter(const event::Enter &e) {
 	Menu *parentMenu = dynamic_cast<Menu*>(parent);
 	if (!parentMenu)
 		return;
@@ -51,7 +51,7 @@ void MenuItem::onEnter(event::Enter &e) {
 	parentMenu->setChildMenu(childMenu);
 }
 
-void MenuItem::onDragDrop(event::DragDrop &e) {
+void MenuItem::onDragDrop(const event::DragDrop &e) {
 	if (e.origin != this)
 		return;
 	doAction();
diff --git a/src/ui/TextField.cpp b/src/ui/TextField.cpp
index b6dc4e38..8873457d 100644
--- a/src/ui/TextField.cpp
+++ b/src/ui/TextField.cpp
@@ -29,14 +29,14 @@ void TextField::draw(NVGcontext *vg) {
 	nvgResetScissor(vg);
 }
 
-void TextField::onButton(event::Button &e) {
+void TextField::onButton(const event::Button &e) {
 	if (e.action == GLFW_PRESS && e.button == GLFW_MOUSE_BUTTON_LEFT) {
 		cursor = selection = getTextPosition(e.pos);
 	}
 	OpaqueWidget::onButton(e);
 }
 
-void TextField::onHover(event::Hover &e) {
+void TextField::onHover(const event::Hover &e) {
 	if (this == context()->event->draggedWidget) {
 		int pos = getTextPosition(e.pos);
 		if (pos != selection) {
@@ -46,11 +46,11 @@ void TextField::onHover(event::Hover &e) {
 	OpaqueWidget::onHover(e);
 }
 
-void TextField::onEnter(event::Enter &e) {
+void TextField::onEnter(const event::Enter &e) {
 	e.consume(this);
 }
 
-void TextField::onSelectText(event::SelectText &e) {
+void TextField::onSelectText(const event::SelectText &e) {
 	if (e.codepoint < 128) {
 		std::string newText(1, (char) e.codepoint);
 		insertText(newText);
@@ -58,7 +58,7 @@ void TextField::onSelectText(event::SelectText &e) {
 	e.consume(this);
 }
 
-void TextField::onSelectKey(event::SelectKey &e) {
+void TextField::onSelectKey(const event::SelectKey &e) {
 	if (e.action == GLFW_PRESS || e.action == GLFW_REPEAT) {
 		switch (e.key) {
 			case GLFW_KEY_BACKSPACE: {