xcb: Don't mark scrolled area as flushed when doing client side scroll

The m_pendingFlush variable is used to track what is missing in the
server side backingstore. If we're doing a client side scroll the
pending area is still the same.

If we were to always discard the scrolled area from m_pendingFlush
we would get in trouble on the next non-client side scroll, as
we think the content exists server-side.

Pick-to: 6.2
Change-Id: Ie50a99a8e5d8a83d1299c53534a1c83c6bfb47bd
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

1 files changed, 9 insertions, 8 deletions

diff --git a/src/plugins/platforms/xcb/qxcbbackingstore.cpp b/src/plugins/platforms/xcb/qxcbbackingstore.cpp
index b4993bfd17..796ca24265 100644
--- a/src/plugins/platforms/xcb/qxcbbackingstore.cpp
+++ b/src/plugins/platforms/xcb/qxcbbackingstore.cpp
@@ -493,10 +493,13 @@ bool QXcbBackingStoreImage::scroll(const QRegion &area, int dx, int dy)
     } else {
         ensureGC(m_xcb_pixmap);
 
-        if (hasShm())
-            shmPutImage(m_xcb_pixmap, m_pendingFlush.intersected(scrollArea));
-        else
+        if (hasShm()) {
+            QRegion partialFlushRegion = m_pendingFlush.intersected(scrollArea);
+            shmPutImage(m_xcb_pixmap, partialFlushRegion);
+            m_pendingFlush -= partialFlushRegion;
+        } else {
             flushPixmap(scrollArea);
+        }
 
         for (const QRect &src : scrollArea) {
             const QRect dst = src.translated(delta).intersected(bounds);
@@ -508,15 +511,13 @@ bool QXcbBackingStoreImage::scroll(const QRegion &area, int dx, int dy)
                           dst.x(), dst.y(),
                           dst.width(), dst.height());
         }
+
+        if (hasShm())
+            m_pendingFlush -= destinationRegion;
     }
 
     m_scrolledRegion |= destinationRegion;
 
-    if (hasShm()) {
-        m_pendingFlush -= scrollArea;
-        m_pendingFlush -= m_scrolledRegion;
-    }
-
     return true;
 }