Mac: Send a window resize event when the window's backing scale factor changes.

The backing scale factor can change when the window moves between retina and non-retina displays.

The only other way to detect such a change is to compare the output of SDL_GL_GetDrawableSize or SDL_GetRendererOutputSize every frame, which is less than desirable, especially since the necessary app logic is likely already being executed when a window resize event is received.

src/video/cocoa/SDL_cocoawindow.h

@@ -70,6 +70,7 @@ typedef enum
 -(void) windowDidDeminiaturize:(NSNotification *) aNotification;
 -(void) windowDidBecomeKey:(NSNotification *) aNotification;
 -(void) windowDidResignKey:(NSNotification *) aNotification;
+-(void) windowDidChangeBackingProperties:(NSNotification *) aNotification;
 -(void) windowWillEnterFullScreen:(NSNotification *) aNotification;
 -(void) windowDidEnterFullScreen:(NSNotification *) aNotification;
 -(void) windowWillExitFullScreen:(NSNotification *) aNotification;

src/video/cocoa/SDL_cocoawindow.m

@@ -255,6 +255,7 @@ SetWindowStyle(SDL_Window * window, unsigned int style)
         [center addObserver:self selector:@selector(windowDidDeminiaturize:) name:NSWindowDidDeminiaturizeNotification object:window];
         [center addObserver:self selector:@selector(windowDidBecomeKey:) name:NSWindowDidBecomeKeyNotification object:window];
         [center addObserver:self selector:@selector(windowDidResignKey:) name:NSWindowDidResignKeyNotification object:window];
+        [center addObserver:self selector:@selector(windowDidChangeBackingProperties:) name:NSWindowDidChangeBackingPropertiesNotification object:window];
         [center addObserver:self selector:@selector(windowWillEnterFullScreen:) name:NSWindowWillEnterFullScreenNotification object:window];
         [center addObserver:self selector:@selector(windowDidEnterFullScreen:) name:NSWindowDidEnterFullScreenNotification object:window];
         [center addObserver:self selector:@selector(windowWillExitFullScreen:) name:NSWindowWillExitFullScreenNotification object:window];
@@ -385,6 +386,7 @@ SetWindowStyle(SDL_Window * window, unsigned int style)
         [center removeObserver:self name:NSWindowDidDeminiaturizeNotification object:window];
         [center removeObserver:self name:NSWindowDidBecomeKeyNotification object:window];
         [center removeObserver:self name:NSWindowDidResignKeyNotification object:window];
+        [center removeObserver:self name:NSWindowDidChangeBackingPropertiesNotification object:window];
         [center removeObserver:self name:NSWindowWillEnterFullScreenNotification object:window];
         [center removeObserver:self name:NSWindowDidEnterFullScreenNotification object:window];
         [center removeObserver:self name:NSWindowWillExitFullScreenNotification object:window];
@@ -584,6 +586,22 @@ SetWindowStyle(SDL_Window * window, unsigned int style)
     }
 }
 
+- (void)windowDidChangeBackingProperties:(NSNotification *)aNotification
+{
+    NSNumber *oldscale = [[aNotification userInfo] objectForKey:NSBackingPropertyOldScaleFactorKey];
+
+    if (inFullscreenTransition) {
+        return;
+    }
+
+    if ([oldscale doubleValue] != [_data->nswindow backingScaleFactor]) {
+        /* Force a resize event when the backing scale factor changes. */
+        _data->window->w = 0;
+        _data->window->h = 0;
+        [self windowDidResize:aNotification];
+    }
+}
+
 - (void)windowWillEnterFullScreen:(NSNotification *)aNotification
 {
     SDL_Window *window = _data->window;