https://bugs.winehq.org/show_bug.cgi?id=39637
--- Comment #6 from Dmitry Timoshkov dmitry@baikal.ru --- (In reply to Dmitry Timoshkov from comment #5)
That activation context belongs to comctl32.dll, probably comctl32 registers the "Button" class with CS_GLOBALCLASS.
I've sent a test that confirms this.