Will investigate this in more detail tomorrow, but I had a series of very odd UI issues relating to full-screen splits on macOS 26.3.1 over the past few days. One example:
An attempt to use Scrivener maximised (full screen), split with Apple Pages. After dragging Scrivener on top of Pages to create a new screen split, Scrivener’s window was entirely black - Pages was fine, but Scrivener was just a black space. Moving the mouse to the top of the screen showed Scriveners menu bar, so it was there, just not rendering.
So I then restored both application windows (cancelled fullscreen) and tried again. It still did not work.
I rebooted my Mac at this point.
Third attempt, and this is when it became really strange: Rather than splitting, Scrivener became confused and attempted to go full screen… but stalled midway, leaving the window partially rendered mid-expansion, so skewed during the 3D transformation. Bizarrely, the application was still working.
Closed Scrivener & Pages (again). Restarted them both. This time in Scrivener, I made sure to use a standard layout. Screen splitting worked fine at this point. I was then able to switch back to my layout in Scrivener, and it’s been fine since.
All very bizarre! I often use Scrivener split with another application (Pages or Apple Notes) and have never seen this before.
MacBook Pro M1 Max / macOS 26.3.1 / Scrivener 3.5.2