[Lazarus] TWinControl.WMEraseBkgnd - should it always set Message.Result := 1

[cobines]

Hello.

The WMEraseBkgnd handler is implemented as:

procedure TWinControl.WMEraseBkgnd(var Message: TLMEraseBkgnd);
begin
  if (Message.DC <> 0) and (wcfEraseBackground in FWinControlFlags) then
  begin
    EraseBackground(Message.DC);
    Message.Result := 1;
  end;
end;

On Windows, if it does not set Result to 1 then, as it says in MSDN:

"The DefWindowProc function erases the background by using the class
background brush specified by the hbrBackground member of the WNDCLASS
structure."

So, the background is painted anyway but it may be painted with a
wrong color, because hbrBackground might be different than
TWinControl.Brush.Color.

This sometimes causes flickering because multiple WM_ERASEBKGND
messages are received during repainting but LCL does not accept some
of them and first Windows repaints background with hbrBackground and
then LCL repaints background with Brush.Color.

Putting Windows aside, from LCL points of view, what should happen if
WM_ERASEBKGND comes in but wcfEraseBackground is not in Flags? Does it
mean background should not be repainted? Or it just should not be
repainted by LCL but can be by the widgetset?

I don't know this is LCL problem (don't allow painting with
hbrBackground of window class) or LCLWin32 problem (set hbrBackground
to NULL for example).

--
cobines