I have this problem too. I found a weird, non-code partial solution:
run some sort of constantly-updating graphical program in the
background, like ico or something like that. I think the problem is
that the X server or window manager goes to sleep and won't wake up
until you get its attention. I also gave the X server real time
priority in windows, but it didn't really help. With ico running in the
background, alt-tab works fine. I didn't have it installed with cygwin,
so I xhosted it back from my linux box.
I hope this is a good enough clue for the X maintainers, I'd look for
differences in the code around scheduling and see if there were any
optimizations related to sleeping.
Thanks,
Tony
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