macOS: Propagate drawRect: dirty bounding rect as fallback

Change-Id: I333e2bfe4a25bfbfebef7b2ec30a600fd441c9a9
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/plugins/platforms/cocoa/qnsview_drawing.mm

@@ -175,10 +175,8 @@
     This method is called by AppKit for the non-layer case, where we are
     drawing into the NSWindow's surface.
 */
-- (void)drawRect:(NSRect)dirtyRect
+- (void)drawRect:(NSRect)dirtyBoundingRect
 {
-    Q_UNUSED(dirtyRect);
-
     Q_ASSERT_X(!self.layer, "QNSView",
         "The drawRect code path should not be hit when we are layer backed");
 
@@ -192,6 +190,9 @@
     for (int i = 0; i < numDirtyRects; ++i)
         exposedRegion += QRectF::fromCGRect(dirtyRects[i]).toRect();
 
+    if (exposedRegion.isEmpty())
+        exposedRegion = QRectF::fromCGRect(dirtyBoundingRect).toRect();
+
     qCDebug(lcQpaDrawing) << "[QNSView drawRect:]" << m_platformWindow->window() << exposedRegion;
     m_platformWindow->handleExposeEvent(exposedRegion);
 }