macostext-editor
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?
I figured out the answer while confirming a few behaviours of the mystery white box while writing the question. I'm posting and answering the question anyway since none of my searches turned up anything.
So it seems it's the File Preview box: if you hit space in the 'Open Files' dialog with a file selected, a similar box gives you an enlarged preview of whatever file is selected.
In the 'Save As' dialog, however, you can't select files (clicking just copies the file name), so it will always come up as 'No items selected' any time it comes up. While you can select folders in the Save As dialog, it doesn't attempt to preview them and continues simply saying that nothing is selected.
Here's a screen grab of it functioning by previewing the screen grab of itself not functioning:
Seems like it can never show anything other than 'No items selected' in the Save As dialog.
I answered a similar question here - cmd-tab behavior on Mavericks with multiple displays
The Task Switcher does follow the Dock. If the Dock is on screen 1, the Task Switcher will appear on screen 1. If the Dock is on screen 3, the Task Switcher will appear on screen 3. Etc.
To make your Dock appear on a screen you can use a couple of methods.
Method 1 - Move your mouse to the bottom of the desired display. Don't stop once you reach the bottom of the display though, try to move it further down, as if you're pushing down on the bottom of the display with your cursor. This action tells the Dock to move to this display. This is temporary however, meaning the Dock will stay on this display until you perform this action on another display or reboot your Mac.
Method 2 - This will change the default starting point for your Dock. In System Preferences > Displays > Arrangement you can drag the menu bar from one display to another in this windows display icons. See the attached picture for reference. This alters the default preference to always show the Dock on the desired display, the one you drag the menu bar to in this preference pane, when you boot and/or login to your user account. You can still use method 1 to temporarily change the Dock's location but upon a reboot it will return to the display specified here.
Best Answer
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.