xcb: Fix flushing of native subwindows when not using shared memory

When using X-forwarding we're not using shared memory for the backing
store image, and end up in a code path where we first copy the updated
parts of the backing store from our client side image over to the server,
and then flush those parts from the server-side image to the window.

The problem was that this code path didn't account for the possibility
that we'd flush a sub-window at an offset, and would end up uploading
the sub-window local region directly, without applying the offset.

This problem was revealed when 79bf1b7e348d started being smarter
about what regions we flush and to what windows when we have sub
windows.

Fixes: QTBUG-81723
Change-Id: I1c9c8bc53c088cdc1ae8b892e17930f4a468ccad
Reviewed-by: Gatis Paeglis <gatis.paeglis@qt.io>
(cherry picked from commit 81e09ae404b632a92e1e4c27f5875bdf027c5401)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit b856b83838947da16664b4fbfa32f76eb01fbca6)
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

1 files changed, 11 insertions, 2 deletions

diff --git a/src/plugins/platforms/xcb/qxcbbackingstore.cpp b/src/plugins/platforms/xcb/qxcbbackingstore.cpp
index 8f55bc2e96..5569bd00cf 100644
--- a/src/plugins/platforms/xcb/qxcbbackingstore.cpp
+++ b/src/plugins/platforms/xcb/qxcbbackingstore.cpp
@@ -710,9 +710,10 @@ void QXcbBackingStoreImage::put(xcb_drawable_t dst, const QRegion &region, const
     Q_ASSERT(!m_clientSideScroll);
 
     ensureGC(dst);
-    setClip(region);
 
     if (hasShm()) {
+        setClip(region); // Clip in window local coordinates
+
         // Copy scrolled area on server-side from pixmap to window
         const QRegion scrolledRegion = m_scrolledRegion.translated(-offset);
         for (const QRect &rect : scrolledRegion) {
@@ -733,7 +734,15 @@ void QXcbBackingStoreImage::put(xcb_drawable_t dst, const QRegion &region, const
         const QRect bounds = region.boundingRect();
         const QPoint target = bounds.topLeft();
         const QRect source = bounds.translated(offset);
-        flushPixmap(region);
+
+        // First clip in backingstore-local coordinates, and upload
+        // the changed parts of the backingstore to the server.
+        setClip(source);
+        flushPixmap(source);
+
+        // Then clip in window local coordinates, and copy the updated
+        // parts of the backingstore image server-side to the window.
+        setClip(region);
         xcb_copy_area(xcb_connection(),
                       m_xcb_pixmap,
                       dst,