It has usually happened to me when running graphics intensive applications through wine but has also occurred when just playing video with mpv.
all graphics other than mouse cursor freezes and TTY canging does not work, mouse cursor does not leave a "trail", but can't interact with anything on the screen.
Everything else seems to work like before, graphical applications do not crash, they still continue normally playing audio if they were before the freeze.
Nothing seems unusual when observed through htop or other tools through SSH.
if I try to startx again it just hangs doing nothing.
if X (from this attempt) is killed, one of its threads goes to D state like before but takes several minutes to die.
when shut down it stalls every time and I have to pull the plug to restart.
Just before making this post, I got a different kind of hang, one I could switch TTYs in. I killed startx with SIGINT, after waiting for several seconds, it said 'X server refuses to die'. I killed X and tried to startx again, which very slowly printed the usual lines, both monitors started displaying a blinking underscore cursor at top left.
Changing TTY the screen appeared frozen (the cursor staying there but without blinking), switching back resulted in it blinking like before.
To my surprise, it somehow worked, the output to VGA monitor dropped and stayed off while the HDMI monitor displayed a "1920x1080 / 60p" box (monitor's builtin message). the same underscore cursor top left stayed there.
With different resolutions the monitor still said 1920x1080.
I have no idea what happened next, SSH connection froze and I couldn't SSH in any more. The screen scrolled messages so fast I could not read them. Music from MPD was still playing but the machine did not respond to any inputs, the WLAN network it hosts shut down and both HTTP and SSH were unreachable through internet.
Two pictures of the fast scrolling screen are at the logs directory.
Please tell me if I didn't provide something that would be of help.