In paintAndFlush, QWidgetRepaintManager subtracts opaque children if
the target isn't overlapped and isMoved is set to true. So in moveRect,
set isMoved to true after the blitting of movable areas, and reset it to
false if we have overlapped sibling or child regions. Otherwise, moving
so far that sourceRect is invalid (none of the original pixels are
visible after the move) we end up in a code path that sets isMoved to
true even with overlapping children or siblings, which then breaks
paintAndFlush's assumptions.
Reuse the test case written by Sergiy Korobov <tiamatenko@gmail.com> in
earlier attempts to fix this bug.
Fixes: QTBUG-26269
Pick-to: 6.2
Change-Id: If7443863f5eee79a80220cd587522122f42a21e4
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
22634e0079