I'm running Version 10 Beta 15 as well as version 2022.1 of IntelliJ. I am having an issue where I can't close an IntelliJ window using the X in the upper right. It seems to be behaving exactly like the restore button. See attached video. The only way I can close the window is to right click on the icon in the Task Bar and then click on Close window. I only have this problem with IntelliJ windows.
This works fine if I close DisplayFusion

I am also having this issue. I downgraded to Display Fusion Pro 9.8 to see if that would help. It did not. I did notice that if I hold down the shift key while clicking on the "X" then the close fuction works correctly. If I move it to the monitor that is not split, it works correctly.

I went ahead and turned on logging. In the hopes that this would help you with troubleshooting.

16:37 - Clicked on close (X) and it performed "maximize"
16:38 - Clicked on close (X) and it performed "Maximize"
16:39 - Held down SHIFT While clicking on close (X) and I got the exit confirmation screen.

I hope this helps!

Here is my debug file. Hit X a few times at 1:52.
And thanks to the previous user about the hint to do a Shift-click to close

I am using IntelliJ IDEA 2022.1 (Ultimate Edition) Build #IU-221.5080.210

Same here: 5080-210

Here you go

What is the best way to display the splits but be able to get them back quickly?

I don't know if it is relevant, but when I have IntelliJ running on the monitor which has no splits, everything works correctly.