[Ubuntu] mouse immediately enters, stays in clicked state on launch

• Mar 10, 2021 - 16:27
Reported version
S2 - Critical

(see: https://musescore.org/en/node/279201)

I currently have three MuseScore 3 instances on my (newish) Dell XPS 13 2-in-1 2020 (7390) [sheesh that's a lot of numbers!], running Xubuntu:
1. 3.2.3 from snapcraft (rarely used due to being out-of-version -- mostly used for debugging issues like these);
2. 3.6.2 from the PPA (used most frequently due to ease of updating); and
3. 3.6.0 as a portable AppImage from the MuseScore site.

Unfortunately, some boots of my machine will randomly cause the following glitch to occur, usually among all three:

App launch and score selection works as normal (the score selection sheet always goes to my "primary" screen instead of where I launched the app, but that's whatever). But, as soon as I'm faced with a score to edit, my cursor immediately gets stuck in the "closed hand" icon, as if I was constantly clicking on the canvas to pan around my score. And, indeed, if I move my mouse, MuseScore reacts as if that were the case, panning. And, if I click (and release!) on any notes and move my mouse, the notes get dragged up and down as if I were clicking and dragging them.

Pressing Esc changes the icon back to the default "pointer" cursor, but does not affect behaviour. Behaviour usually persists between reboots, and if it happens to one Musescore instance it'll usually happen within a couple of tries on other instances as well. Some [EDIT: app restarts] have this issue, some don't. I primarily use my laptop connected to an external monitor and keyboard/mouse, but the behaviour persists using the trackpad and built-in laptop keys as well, as well as if I restart/disconnect-reconnect these peripherals.

The machine is dual-booted with Windows, but my workflow completely prevents me from using the Windows partition for MuseScore, so it's between using the Ubuntu version to write or nothing at all.

This ain't my first bug-fixing rodeo (I actually wanted to use Musescore while procrastinating on fixing bugs at work, lol) so happy to provide any screengrabs, technical details, repros, logs, etc.

Thanks, and I look forward to getting back to using Musescore fearlessly!