The problem you're addressing

Currently, many scripts and users and rolling ad-hoc methods of determining the currently focused VM. For example:

1. For the i3 window manager, @marmarek added a function to do this
2. Sven's “poor man’s” pre-loaded DispVM

and I've seen this used in other places too.

The solution you'd like

A new script /usr/bin/qubes-focused-vm:

#!/bin/bash

# get ID of currently focused window
id=$(xprop -root _NET_ACTIVE_WINDOW)
id=${id##* }

vm=$(xprop -id $id | grep '_QUBES_VMNAME(STRING)')

if [ "$vm$ == "" ]; then
    echo "dom0"
    # dom0 does not set _QUBES_VMNAME
else
    # extract VM name
    vm=${vm#*\"} 
    echo ${vm%\"*}
fi

Note that I have added the if statement for forward compatibility in case dom0 ever sets _QUBES_VMNAME (why doesn't it, by the way?). It assumes dom0 is the only VM that doesn't set _QUBES_VMNAME.

The value to a user, and who that user might be

This simplifies various existing scripts. Also, users may want to add their own shell scripts that bind to a hotkey and perform some action on the currently focused VM. Having a script like this greatly simplifies readability. For example, in my i3 config, I have:

bindsym $mod+Shift+Return exec qrexec-client -e -d "$(qubes-focused-vm)" 'DEFAULT:alacritty -e ranger'

which would by a total mess without the above script.

In fact, it may be worth creating a qubes-run-in-focused-vm script too.

I'd be happy to submit a pull request if you just let me know where's the best place. Possibly here?