Hi Michel and Martin - for the full screen mode sliding away that hint Michel gave about it only happening when moving over a viewport was very helpful. It turns out that it's caused by the little view controls panel window that's shown at the bottom of viewports. That little window is getting shown back on the original desktop space and when that happens the OS zooms you back there again. Now that I know what's happening I can work on a fix, thanks!

If you turn off the view controls panel under Options > View > "Show view controls" you should see that you won't get sent back to the desktop in full screen mode anymore.

- Michael