Hi,
FWIW I am unable to reproduce on Win 7
Pro, Py 3.5.1, PyGame 1.9.2a0 (to print the latter,
`print(pygame.ver)`).
The event system, like everything else,
is really just a thin layer over SDL's—so for
platform-specific issues in PyGame it's commonly the
underlying SDL that's at fault. Notwithstanding that SDL is
fairly well-tested, but Mac tends to be a lower priority for
testing, on account of Apple making game development on OS X
difficult in a variety of ways. My guess is that it's
a minor hiccup in older SDL on MacOS that no one ever noticed
before, additionally because MOUSEBUTTONUP is often ignored—in
which case you're most likely to get closure from the SDL
community, unless someone here feels like C spelunking.
Maybe `pygame.event.clear()` would trick
the event system into behaving better on startup?
Ian