Avoid re-rendering rails when scrolling with a framebuffer margin

src/app/RackWidget.cpp

@@ -16,6 +16,7 @@ namespace rack {
 
 RackWidget::RackWidget() {
 	rails = new FramebufferWidget();
+	rails->box.size = Vec();
 	{
 		RackRail *rail = new RackRail();
 		rail->box.size = Vec();
@@ -342,17 +343,20 @@ void RackWidget::step() {
 	// Adjust size and position of rails
 	Widget *rail = rails->children.front();
 	Rect bound = getViewport(Rect(Vec(), box.size));
-	// bound.pos = bound.pos.plus(Vec(100, 100));
-	// bound.size = bound.size.minus(Vec(200, 200));
-	rails->box = bound;
-	Vec grid = Vec(RACK_GRID_WIDTH, RACK_GRID_HEIGHT);
-	Vec gridPos = bound.pos.div(grid).floor().mult(grid);
-	bound.pos = gridPos.minus(bound.pos);
-	bound.size = bound.size.minus(bound.pos);
-	if (!bound.isEqual(rail->box)) {
+	if (!rails->box.contains(bound)) {
+		// Add a margin around the otherwise tight bound, so that scrolling slightly will not require a re-render of rails.
+		Vec margin = Vec(100, 100);
+		bound.pos = bound.pos.minus(margin);
+		bound.size = bound.size.plus(margin.mult(2));
+		rails->box = bound;
+		// Compute offset of rail within rails framebuffer
+		Vec grid = Vec(RACK_GRID_WIDTH, RACK_GRID_HEIGHT);
+		Vec gridPos = bound.pos.div(grid).floor().mult(grid);
+		bound.pos = gridPos.minus(bound.pos);
+		bound.size = bound.size.minus(bound.pos);
+		rail->box = bound;
 		rails->dirty = true;
 	}
-	rail->box = bound;
 
 	// Autosave every 15 seconds
 	if (gGuiFrame % (60*15) == 0) {