macOS: Don't optimize out aggressive backingstore flushes when single-buffered

Comparing the layer contents to the backingstore surface doesn't make
sense when we're single buffered since we're always using the same
surface.

And once Core Animation has picked up on the surface it's not enough to
just keep drawing to it, we also need to tell Core Animation that the
contents has changed.

Change-Id: I517a0b7a3ba7e9d96033465c9bd5a192985643ac
Fixes: QTBUG-81071
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

1 files changed, 4 insertions, 2 deletions

diff --git a/src/plugins/platforms/cocoa/qcocoabackingstore.mm b/src/plugins/platforms/cocoa/qcocoabackingstore.mm
index 3b9df8da3a..ba5d42e706 100644
--- a/src/plugins/platforms/cocoa/qcocoabackingstore.mm
+++ b/src/plugins/platforms/cocoa/qcocoabackingstore.mm
@@ -537,8 +537,10 @@ void QCALayerBackingStore::flush(QWindow *flushedWindow, const QRegion &region,
         flushedView.layer.contentsScale = m_buffers.back()->devicePixelRatio();
     }
 
+    const bool isSingleBuffered = window()->format().swapBehavior() == QSurfaceFormat::SingleBuffer;
+
     id backBufferSurface = (__bridge id)m_buffers.back()->surface();
-    if (flushedView.layer.contents == backBufferSurface) {
+    if (!isSingleBuffered && flushedView.layer.contents == backBufferSurface) {
         // We've managed to paint to the back buffer again before Core Animation had time
         // to flush the transaction and persist the layer changes to the window server, or
         // we've been asked to flush without painting anything. The layer already knows about
@@ -555,7 +557,7 @@ void QCALayerBackingStore::flush(QWindow *flushedWindow, const QRegion &region,
     // with other pending view and layer updates.
     flushedView.window.viewsNeedDisplay = YES;
 
-    if (window()->format().swapBehavior() == QSurfaceFormat::SingleBuffer) {
+    if (isSingleBuffered) {
         // The private API [CALayer reloadValueForKeyPath:@"contents"] would be preferable,
         // but barring any side effects or performance issues we opt for the hammer for now.
         flushedView.layer.contents = nil;