Patching Win32Proc.cpp line #71 with

bool GetMouseLeft()   { AvoidPaintingCheck__(); return !!(GetKeyState(VK_LBUTTON) & 0x8000); }

resolved the problem, in spite of having added loops like

for(int i = 0; i < 10000; i++)
    GetMouseLeft();

to both ToolButton::Paint and Button::Paint. In other words, I was able to 'switch' problem on/off just by removing/adding patch above, respectively.

Just don't know about eventual (unforeseen) consequences (if any) of patch to Upp code as a whole...

tks!