Yep! I'm afraid it does! Updating windows library hasn't worked. Actually, it happens and disappears just like it comes. I'm working on the hypothesis that some other process might be messing with user32.dll. I'll try to track which process(es) is(are) using it and block its(their) execution to see what happens. Any suggestions?
What about putting a big loop doing just GetMouseLeft into ToolButton::Paint? Then try in some more trivial situation (like single normal Button::Paint).