i have done a bit more testing. here are some additional notes:
i have found a reliable way to trigger the issue. i am using firefox-esr.
to cause the x-freeze issue, i can play a youtube video. and as it is playing, i repeatedly move the mouse over the video, and then off of it. this causes the "play controls" at the bottom of the youtube video to appear and disappear repeatedly. eventually this triggers the problem. the mouse pointer gets sluggish, the video starts to stutter and eventually stops updating all together. the mouse gets very sluggish and after a while will not move at all. sometimes takes several minutes of moving the mouse to move 1cm on the screen -- that sort of thing.
during this time the keyboard seems to remain responsive: i can ctrl+alt+f2 into a terminal, and although i cannot see the terminal because the x screen is frozen still, i can log in as a user, run commands that seem to execute immediately, and if i hold the backspace key i get repeated rapid beeps from the computer. this suggests to me that the keyboard and the rest of the system remains responsive, and only x is hung/frozen.
since i am now able to trigger this issue at will, i have tested several times where i log in to terminal, run a command, and then reboot with ctrl+alt+del. i have done this several times now and not triggered the "unable to log in after screen freeze" issue, so that part of it remains intermittent -- i am USUALLY able to log in after the x-freeze without interventions. twice now however i have been unable to until i modified permissions (ownership) on files in my home directory, as described previously in this bug report.
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