Weird bar in the bottom-left. What is that? Sometimes, when I hit a combination of keys (I have not figured out which one) a weird bar in the bottom-left of a screen appears. I cannot get rid of it in any way. The screencast: https://youtu.be/CdXVgth_sTk

The only way I have figured out of removing it is via: killall Dock.

What is the bar in question? How do you trigger it, how do you get rid of it, how to use it?
Use QuickTime to record your screen with the MacOS onscreen keyboard visible and you'll be able to figure out what key combo causes this to appear. Then you can track it down to a particular package.