Widgets: Use accelerated scroll when scrolled widget is overlapped

Get region of overlapped widgets and scroll only non-overlapped parts
of image. Next, schedule an update for overlapped widgets region.

This patch improves scrolling performance when scrolled widget has
overlapped widgets.

Common use cases:
- faster scrolling when using "StyleHint::SH_ScrollBar_Transient",
- faster scrolling of zoomed image with semi-transparent thumbnail.

Accelerated scrolling with overlapped widgets is not available when
scale factor is non-integer.

Task-number: QTBUG-64504
Change-Id: I8337d3bc756e50f7d31cdc7979ccf86dc5c3695f
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/widgets/kernel/qwidget_p.h b/src/widgets/kernel/qwidget_p.h
index c39333161b..fae9e5ff4e 100644
--- a/src/widgets/kernel/qwidget_p.h
+++ b/src/widgets/kernel/qwidget_p.h
@@ -453,7 +453,7 @@ public:
     // ### Qt 4.6: Merge into a template function (after MSVC isn't supported anymore).
     void invalidateBuffer(const QRegion &);
     void invalidateBuffer(const QRect &);
-    bool isOverlapped(const QRect&) const;
+    QRegion overlappedRegion(const QRect &rect, bool breakAfterFirst = false) const;
     void syncBackingStore();
     void syncBackingStore(const QRegion &region);