tdf#101978 vcl combobox/listbox floating window: avoid flicker

Regression from commit dca01def7885ad69cf66edd75cf8207a5adb64f9
(refactor ListBox/ComboBox to use RenderContext, 2015-05-07), the
problem was that with VCL backends which don't paint on idle, the widget
flickered on keyboard / mouse interaction, similar to the Calc
autofilter.

To avoid side-effects:

- Avoid direct paint in ScrollBar::ImplDragThumb(), instead do
Invalidate() + Update(), which is the same, but this way dragging the
scrollbar with the mouse doesn't fails an assertion (because we tried to
paint directly on a double-buffered window).

- Only call RequestDoubleBuffering() for backends which don't handle
focus themselves (i.e. not for gtk3); to be in sync with the Calc
autofilter behavior.

Change-Id: Iec9ad2fe9d32efb8da65cc26cf89cc47912bf545
Reviewed-on: https://gerrit.libreoffice.org/74176
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/vcl/source/control/scrbar.cxx b/vcl/source/control/scrbar.cxx
index 19db246006b7..b20711d5e2f8 100644
--- a/vcl/source/control/scrbar.cxx
+++ b/vcl/source/control/scrbar.cxx
@@ -816,7 +816,13 @@ void ScrollBar::ImplDragThumb( const Point& rMousePos )
             // When dragging in windows the repaint request gets starved so dragging
             // the scrollbar feels slower than it actually is. Let's force an immediate
             // repaint of the scrollbar.
-            ImplDraw(*this);
+            if (SupportsDoubleBuffering())
+            {
+                Invalidate();
+                Update();
+            }
+            else
+                ImplDraw(*this);
 
             mnDelta = mnThumbPos-nOldPos;
             Scroll();