macOS: Don't scroll individual rects of a region when inline scrolling

The QPlatformBackingStore::scroll() API takes a QRegion as input, and we
kept this data type in the implementation of QCALayerBackingStore::scroll
for the logic of figuring out whether we could use the existing back
buffer as source of the scroll or if we needed to pull data from the
front buffer.

We then iterated the rects of the resulting in-line region, and called
qt_scrollRectInImage for each of them. This will unfortunately not work
whenever the scroll of one of these rects ends up overwriting parts of
the back buffer that later rects of the region use as their source rect.

This could be reproduced by moving the cursor within a text document,
which would dirty a small area around the cursor, making the subsequent
scroll of the viewport result in garbled text.

This issue exists in the other platform backingstore implementations
too, but since the QWidgetRepaintManager never calls scroll() with
anything but a single rect they would not have any issues.

The problem could potentially be solved by sorting the rects before
blitting them, but now this quick fix solves the regression on macOS.

Fixes: QTBUG-102181
Pick-to: 6.2 6.3 6.3.0
Change-Id: I80f6d26117321e3c09e068cdb03eb320e244a5de
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

1 files changed, 10 insertions, 8 deletions

diff --git a/src/plugins/platforms/cocoa/qcocoabackingstore.mm b/src/plugins/platforms/cocoa/qcocoabackingstore.mm
index 8f3e3036ad..a8e6791e84 100644
--- a/src/plugins/platforms/cocoa/qcocoabackingstore.mm
+++ b/src/plugins/platforms/cocoa/qcocoabackingstore.mm
@@ -271,20 +271,22 @@ bool QCALayerBackingStore::scroll(const QRegion &region, int dx, int dy)
     m_buffers.back()->lock(QPlatformGraphicsBuffer::SWWriteAccess);
 
     if (!inPlaceRegion.isEmpty()) {
-        qCDebug(lcQpaBackingStore) << "Scrolling" << inPlaceRegion << "in place";
+        // We have to scroll everything in one go, instead of scrolling the
+        // individual rects of the region, as otherwise we may end up reading
+        // already overwritten (scrolled) pixels.
+        const QRect inPlaceBoundingRect = inPlaceRegion.boundingRect();
+
+        qCDebug(lcQpaBackingStore) << "Scrolling" << inPlaceBoundingRect << "in place";
         QImage *backBufferImage = m_buffers.back()->asImage();
         const qreal devicePixelRatio = backBufferImage->devicePixelRatio();
         const QPoint devicePixelDelta = scrollDelta * devicePixelRatio;
 
         extern void qt_scrollRectInImage(QImage &, const QRect &, const QPoint &);
 
-        for (const QRect &rect : inPlaceRegion) {
-            qt_scrollRectInImage(*backBufferImage,
-                QRect(rect.topLeft() * devicePixelRatio,
-                      rect.size() * devicePixelRatio),
-                      devicePixelDelta);
-        }
-
+        qt_scrollRectInImage(*backBufferImage,
+            QRect(inPlaceBoundingRect.topLeft() * devicePixelRatio,
+                  inPlaceBoundingRect.size() * devicePixelRatio),
+                  devicePixelDelta);
     }
 
     if (!frontBufferRegion.isEmpty()) {