This is similar to the full-screen bug reported in post https://forum.literatureandlatte.com/t/lh3648-issue-when-returning-from-composition-mode/49168/1 . It appears that that bug has been fixed, but only partially.
If Scrivener is not in full screen mode, then you enter Composition Mode and return to normal mode, the original window is shorter and narrower than it was when entering Composition Mode.
This bug still exists for me, irrespective of whether Scrivener is maximised.
To be clear, in Scrivener 3.1.1, after entering and exiting composition mode, the main project window has reduced its size? Could you share the steps to reproduce the problem and what OS version you’re on? I haven’t been able to reproduce the issue now on Windows 10 or 11 following the steps that previously triggered it (such as having the side of the main window touching the edge of the screen), so there may be something particular to the window position or other settings that is necessary to see this.
I can no longer duplicate this, either, using 3.1.10. I have a 3-monitor setup and no matter which monitor I placed the main window on before entering Composition Mode, and regardless of where on the monitor the window was positioned, it returned to the original size and position.
But if only about 1/3 to 1/4 of the original window is visible on the far right of the rightmost monitor, entering Composition Mode causes the whole window to disappear, though the app is still running. And the app icon on the task bar goes back to its “the program is not running” appearance. Pressing ESC brings the app back into view. This does not happen on the leftmost monitor.
I am unable to reproduce it currently. But there have been no changes to my system since 3 days ago when the problem occurred. I will have to wait until it happens again and post a screencast of it. This is on Windows 11 with v3.1.1.