Keyboard use (whether typing, switching windows with alt+tab) causes a delay before two-finger scrolling works again. Actions like moving the pointer and clicking work immediately. This does not appear related to a movement threshold, because wildly exaggerated movements fail to cause two-finger scroll to register. Delay seems to be at least one full second, perhaps longer.
weird. the timeout for dwt after multiple key presses is 500ms, so any timeout should not be longer than that. There is nothing specific about scrolling here, if movement works then gestures should work too. Please run libinput-debug-events --verbose and try to reproduce this issue, then attach the output here. Maybe there's some bug in the state transition you're triggering. The only legitimate case where this may be triggered is if you put the first finger down while still typing, stop typing and then put the second finger down to scroll. The first finger will be identified as palm and continue to be identified as such - you'd get pointer motion but no 2fg scrolling.
Whoops! Forgive the oversight -- the observed behavior did not occur in a session running libinput. libinput is indeed working as expected. Apologies for the sloppy bug report.
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