Make mupdf viewer option auto-update

[user202729]

Is your feature request related to a problem? Please describe it.
Currently if mupdf is used as a viewer, it will not automatically refresh when the PDF file change.

Describe the solution you'd like
Implement it. It can be done by sending HUP signal to the mupdf process https://unix.stackexchange.com/questions/425907/how-to-make-mupdf-automatically-refresh-a-document

At least just spawning kill -HUP <pid> work in UNIX environments, not sure about Windows.

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Edit: we can use job_stop() in vim to signal SIGHUP, but still not sure if it works on Windows.

[lervag]

Notice that this should work currently if you have xdotool:

vimtex/autoload/vimtex/view/mupdf.vim

Line 29 in a4d5e2f

call self.xdo_send_keys('r')

You are asking for an adjustment where we instead signal mupdf directly. It seems much less of a hack, and so I think it could be interesting to consider it.

To implement, we would need:

1. Use vimtex#jobs#start to start the viewer, as that will use the modern jobstart or job_start apis and gives access to PIDs and possibly signalling.
2. Either signal with kill -HUP PID or perhaps by use of built-in job_stop (there does not seem to be a corresponding version on neovim).

[user202729]

Ah, for some reason it didn't work earlier (even though I have xdotool). I changed view method back to mupdf and it works now..

[lervag]

I've made the changes now, and it seems to work well. Please test - does it work for you?

[user202729]

That's fast. Yes, it works.

[lervag]

Great, glad to hear it. And thanks for the idea, I think this should be a general improvement overall for the mupdf layer.