key "stuck down" after sendevent

[Julian Bradfield]

I have a puzzling (to me) observation.

In the course of marking lots of exercise, I'm doing, repeatedly,

mupdf -geometry +0+0 *.pdf &  emacs q3.txt ; sleep 0.1; xdotool search
.pdf key q 

The purpose of the xdotool is so that mupdf receives a quit command
and saves its history file, rather than being killed. (The sleep 0.1
(using a local sleep that accepts floating point) is to address another
issue.)

Almost invariably, xdotool gets a BadWindowError when it tries to send
the KeyUp event, presumably because by then mupdf has already quit and
accepts no more events.

Sometimes, something weird happens: the window that then has focus
(usually an xterm or emacs) gets auto-repeating q's until I press
and release another key.

I don't see how sending a synthetic keydown to a client can affect the
state of the keyboard for other clients.

X.Org X Server 1.20.13 running on Ubuntu 20.04.4 LTS.