Why does framebufferUpdateRequest(incremental=1) cause expectScreen() to fail?

[reticulatedpines]

framebufferUpdateRequest(incremental=1), used internally by expectScreen(), always fails. There's a cryptic note by alex about this in the old vnc patch, which claims it was intermittent then.

I use _fbReplacer() to work around this without requiring upstream changes, but it's kind of ugly. Can we understand this problem so we can get upstream to fix it, or find a real fix our side? Might be a qemu interaction, if we could get a clean repro on vanilla qemu with some standard OS, that would probably be enough to open a useful issue with vncdotool.

See 43bd315