Apple - Invisible region on screen is blocking mouse cursor

I had this same problem and it has been driving me crazy. While I still don’t fully understand the cause of the bug I finally found a way to fix it.

First a note, I think this problem only happens when the zoom accessibility mode is enabled, though it occurs even when not zoomed in. And further, it might be related exclusively to full screen zoom.

I have a keyboard shortcut that turns full screen zoom on and off (⌘⌥8 by default), configured in accessibility settings. Whenever a region of one of my displays becomes inaccessible, I just zoom in and right back out and the inaccessible regions are gone.

I hope this helps.

Move the white bar at the top of the smaller display to the larger display. I had the exact same issue (27" monitor and 24" monitor). I was able to move the "box" up and down the right side of my "primary" monitor, by sliding one display up and down on the arrangement tab.

Then I Googled how to select your primary display on macOS:

https://support.apple.com/en-us/HT202351

The invisible box is gone now. I think the issue is when you set a smaller display as the primary monitor, there is some gap created near one corner that basically equals the number of mixing pixels between the two monitors. But only at one particular corner, not all of the mixing pixels. I would have had a nice long invisible rectangle + an invisible box if that were the case (2560x1440 > 1920x1080).