How to refresh the client rectangle

Problem/Question/Abstract:

A config form allows the user to change the background, but I can't refresh the complete client area when coming back from the config form. Only the config form area is refreshed.

Answer:

procedure TMainWin.InvalidateClient;
var
  R: TRect;
begin
  R := Classes.Rect(0, 0, ClientWidth, ClientHeight);
  InvalidateRect(ClientHandle, @R, true);
end;