I fear it was nothing that entertaining: it was just my “normal” dark panel at the top of the screen and a second “default” white one at the bottom (this last one partially covered the windows I had open). I didn’t try triggering notifications or otherwise causing some kind of mayhem.
2 more cents :)
I’ve been using syncthing for a while now, on different devices, and the only unreliability I’ve run into is with android killing syncthing to save battery life, which is kinda hilarious, considering all the vendor- and google-provided crap they happily waste battery on (I don’t use it, but for what I’ve heard iOS is even worse in this regard).
Specifically, I have a samsung tablet where, no matter how much I tinkered with system settings, synchthing would only run if I manually launched the app or while the tablet was charging (BTW I still use that same tablet, but it now runs LineageOS and syncthing works flawlessly).
All this is to say, you should probably look into system settings and research ways to convince your OS to do what it’s supposed to rather than tinkering with syncthing itself.