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macOS: Gracefully handle devicePixelRatio mismatch in QCALayerBackingStore

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If the client of the backingstore fails to pick up dpr changes, and
tries to flush the backingstore without a repaint, we will end up
flushing a back-buffer with a stale dpr. Detect when this happens,
warn the user, and smooth out the situation by adjusting the layer
accordingly.

Change-Id: If4596a8976a3902252c81d8e28c7aeb9fdd908bf
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>
This commit is contained in:
Tor Arne Vestbø 2019-03-21 15:26:45 +01:00
parent 585150e3d9
commit 2b2133f853
2 changed files with 21 additions and 3 deletions
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1
src/plugins/platforms/cocoa/qcocoabackingstore.h
View File

@ -93,6 +93,7 @@ private:
QRegion dirtyRegion; // In unscaled coordinates
QImage *asImage();
qreal devicePixelRatio() const { return m_devicePixelRatio; }
private:
qreal m_devicePixelRatio;

23
src/plugins/platforms/cocoa/qcocoabackingstore.mm
View File

@ -460,12 +460,29 @@ void QCALayerBackingStore::flush(QWindow *flushedWindow, const QRegion &region,
NSView *backingStoreView = static_cast<QCocoaWindow *>(window()->handle())->view();
NSView *flushedView = static_cast<QCocoaWindow *>(flushedWindow->handle())->view();
// If the backingstore is just flushed, without being painted to first, then we may
// end in a situation where the backingstore is flushed to a layer with a different
// scale factor than the one it was created for in beginPaint. This is the client's
// fault in not picking up the change in scale factor of the window and re-painting
// the backingstore accordingly. To smoothing things out, we warn about this situation,
// and change the layer's contentsScale to match the scale of the back buffer, so that
// we at least cover the whole layer. This is necessary since we set the view's
// contents placement policy to NSViewLayerContentsPlacementTopLeft, which means
// AppKit will not do any scaling on our behalf.
if (m_buffers.back()->devicePixelRatio() != flushedView.layer.contentsScale) {
qCWarning(lcQpaBackingStore) << "Back buffer dpr of" << m_buffers.back()->devicePixelRatio()
<< "doesn't match" << flushedView.layer << "contents scale of" << flushedView.layer.contentsScale
<< "- updating layer to match.";
flushedView.layer.contentsScale = m_buffers.back()->devicePixelRatio();
}
id backBufferSurface = (__bridge id)m_buffers.back()->surface();
if (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.
// The layer already knows about the back buffer, and we don't need to re-apply
// it to pick up the surface changes, so bail out early.
// 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
// the back buffer, and we don't need to re-apply it to pick up any possible surface
// changes, so bail out early.
qCInfo(lcQpaBackingStore).nospace() << "Skipping flush of " << flushedView
<< ", layer already reflects back buffer";
return;