https://bugs.winehq.org/show_bug.cgi?id=36734
--- Comment #6 from Johannes wine@sagagames.de --- I'm the OpenMPT developer who applied a workaround in OpenMPT for this issue when I read this report, so any OpenMPT version later than the one linked above are not affected, but the actual bug still exists in Wine 2.4 (which is the latest that is currently available through the stable packet repository as it seems).
I don't think there is any more information I can attach since the issue has already been described in full detail in comment #4. The mouse cursor is supposed to become instantly visible after leaving the logo are in the About window, but it only becomes visible after the timeout is reached, which is not supposed to affect the TME_LEAVE event.