OpenGL: Ensure that GL views display at the correct scale on macOS 12

Previously, we were using the window's top-left position to determine
the scale to use for the OpenGLContext. However, on macOS the
backingScaleFactor of the window is not strictly related to the top-left
corner of the window, so the OpenGL view's scale could end up differing
from the backing scale factor when slowly moving a window between
displays with different backing scale factors.

On macOS, we now use the backing scale factor of the window's screen (as
maintained by AppKit), rather than trying to work out the correct
display and scale ourselves.

modules/juce_opengl/opengl/juce_OpenGLContext.cpp

@@ -161,13 +161,6 @@ public:
 
     void resume()
     {
-       #if JUCE_MAC
-        initialDisplayTopLeft = Desktop::getInstance().getDisplays()
-                                                      .getDisplayForRect (component.getTopLevelComponent()->getScreenBounds())
-                                                      ->totalArea
-                                                      .getTopLeft();
-       #endif
-
         if (renderThread != nullptr)
             renderThread->addJob (this, false);
     }
@@ -314,23 +307,26 @@ public:
 
         if (auto* peer = component.getPeer())
         {
-            auto localBounds = component.getLocalBounds();
-            const auto currentDisplay = Desktop::getInstance().getDisplays().getDisplayForRect (component.getTopLevelComponent()->getScreenBounds());
-
-            if (currentDisplay != lastDisplay)
+           #if JUCE_MAC
+            const auto displayScale = [this]
             {
-               #if JUCE_MAC
-                if (cvDisplayLinkWrapper != nullptr)
+                if (auto* wrapper = cvDisplayLinkWrapper.get())
                 {
-                    cvDisplayLinkWrapper->updateActiveDisplay (currentDisplay->totalArea.getTopLeft());
-                    nativeContext->setNominalVideoRefreshPeriodS (cvDisplayLinkWrapper->getNominalVideoRefreshPeriodS());
+                    if (auto* screen = wrapper->updateAndReturnActiveDisplay())
+                    {
+                        nativeContext->setNominalVideoRefreshPeriodS (wrapper->getNominalVideoRefreshPeriodS());
+
+                        return [screen backingScaleFactor];
+                    }
                 }
-               #endif
-                lastDisplay = currentDisplay;
-            }
 
-            const auto displayScale = currentDisplay->scale;
+                return scale;
+            }();
+           #else
+            const auto displayScale = Desktop::getInstance().getDisplays().getDisplayForRect (component.getTopLevelComponent()->getScreenBounds())->scale;
+           #endif
 
+            auto localBounds = component.getLocalBounds();
             auto newArea = peer->getComponent().getLocalArea (&component, localBounds).withZeroOrigin() * displayScale;
 
            #if JUCE_WINDOWS && JUCE_WIN_PER_MONITOR_DPI_AWARE
@@ -590,7 +586,7 @@ public:
 
        #if JUCE_MAC
         cvDisplayLinkWrapper = std::make_unique<CVDisplayLinkWrapper> (*this);
-        cvDisplayLinkWrapper->updateActiveDisplay (initialDisplayTopLeft);
+        cvDisplayLinkWrapper->updateAndReturnActiveDisplay();
         nativeContext->setNominalVideoRefreshPeriodS (cvDisplayLinkWrapper->getNominalVideoRefreshPeriodS());
        #endif
 
@@ -709,13 +705,9 @@ public:
     OpenGLFrameBuffer cachedImageFrameBuffer;
     RectangleList<int> validArea;
     Rectangle<int> viewportArea, lastScreenBounds;
-    const Displays::Display* lastDisplay = nullptr;
     double scale = 1.0;
     AffineTransform transform;
     GLuint vertexArrayObject = 0;
-   #if JUCE_MAC
-    Point<int> initialDisplayTopLeft;
-   #endif
 
     StringArray associatedObjectNames;
     ReferenceCountedArray<ReferenceCountedObject> associatedObjects;
@@ -752,15 +744,30 @@ public:
             return 0.0;
         }
 
-        void updateActiveDisplay (Point<int> topLeftOfDisplay)
+        NSScreen* getCurrentScreen() const
         {
-            CGDirectDisplayID displayID;
-            uint32_t numDisplays = 0;
-            constexpr uint32_t maxNumDisplays = 1;
-            CGGetDisplaysWithPoint (convertToCGPoint (topLeftOfDisplay), maxNumDisplays, &displayID, &numDisplays);
+            if (auto* peer = cachedImage.component.getPeer())
+                if (auto* view = static_cast<NSView*> (peer->getNativeHandle()))
+                    if (auto* window = [view window])
+                        return [window screen];
+
+            return nullptr;
+        }
+
+        /*  If updated, returns the new screen, or nullptr otherwise. */
+        NSScreen* updateAndReturnActiveDisplay()
+        {
+            auto* oldScreen = std::exchange (currentScreen, getCurrentScreen());
+
+            if (oldScreen == currentScreen)
+                return nullptr;
+
+            for (NSScreen* screen in [NSScreen screens])
+                if (screen == currentScreen)
+                    if (NSNumber* number = [[screen deviceDescription] objectForKey: @"NSScreenNumber"])
+                        CVDisplayLinkSetCurrentCGDisplay (displayLink, [number unsignedIntValue]);
 
-            if (numDisplays == 1)
-                CVDisplayLinkSetCurrentCGDisplay (displayLink, displayID);
+            return currentScreen;
         }
 
         ~CVDisplayLinkWrapper()
@@ -783,6 +790,7 @@ public:
         CachedImage& cachedImage;
         const bool continuousRepaint;
         CVDisplayLinkRef displayLink;
+        NSScreen* currentScreen = nullptr;
     };
 
     std::unique_ptr<CVDisplayLinkWrapper> cvDisplayLinkWrapper;