Keywords: OSX Catalina, Macintosh, hide menu bar, display, screen, shift, feature, disable.
TLDR: It’s an accessibility feature called Zoom. Look in System Preferences… Accessibility… Zoom and disable any gestures or keyboard shortcuts pertaining to Zoom.
As soon as I upgraded my newer Mac to Catalina, it started happening: whenever the mouse cursor got close to the top or the bottom of the screen, the display would shift slightly, by 20 or 30 pixels or so.
It lent an overall air of sloppiness to the whole user experience, yet it was obviously an intentional (mis)feature of some sort, because implementing it is non-trivial in code (it requires moving a lot of data around in video memory). There simply was no conceivable way this could happen as the result of a common coding bug. Finally, it had never happened to me before I upgraded to Catalina, and now it always happened, but only on the newer Mac that ran Catalina. The old Mac (which cannot be upgraded, due to it no longer being a supported product) simply never developed this behavior.
So I started looking through the system preferences for the obnoxious new feature. It wasn’t in the “General” or “Desktop & Screen Saver” sections, and I couldn’t see any other obvious place where it might be; nothing else obviously controlled a display issue like this.
The next step was attempting to find an answer via a search engine, but I also kept coming up dry. I gave up, having pissed away well over an hour on the issue by that time, and decided to try living with the misfeature.
But it was annoying, extremely annoying. I like to keep track of the time by looking at the digital clock on the right-hand side of the menu bar, yet the misfeature meant that about half of the menu bar was not visible, which typically made the clock illegible. I could address this by moving the mouse cursor up to the top of the screen, but it’s annoying to have to do that. I shouldn’t have to mess with my pointing device just to see the time of day.
So, I kept revisiting the issue, hoping to come up with the magic keyword that would eventually come up with the solution. Nothing ever worked.
Eventually, I broke down and posted something to Reddit, making sure to be irate and whiny (past experience has shown that an irate tone is more likely to generate responses for such questions).
Sure enough, it was a deliberate feature, one related to an accessibility (for the disabled) feature called, of all things, “zoom,” which is why I had been unable to locate it, or even find out about it via a search. I would have never guessed that shifting the screen like that had anything to do with zooming or magnifying the screen.
So many modern user interface design techniques come across as completely bizarre and counterintuitive to me. I don’t think OSX would even be a usable GUI to me, were it not for how I’ve disable feature after feature in it in the settings over the years.