hey hangs with offscreen windows

[mmu_man]

When writing dump_windows.sh which iterates over all apps and windows, I noticed hey locked up when asking Pe for one of its windows' frame. It seems it's actually an offscreen window which I suppose doesn't run an event loop.

It should probably have a large timeout (2s?) instead of the default infinite timeout. But still having unresponding windows might cause other trouble (like, screen readers which actually must be very responsive), so we might as well want to fix this too.