I'm using Rustrover for a year now on KDE and I can't observe the performance decrease or the AI craze reported here. Neither have I witnessed any impactful bugs like OP describes. In fact I'd say I haven't seen much bugs at all.
I'm not using the AI/LLM features at all and I can't remember if I disabled them or if that's even possible but when I read here that JetBrains is overly focussed on AI, I'd say that you shouldn't dare to leave your carefully crafted nvim environment and take a peek at what's happening over in the vs-code world as their LLM commitment on a whole 'nother level.
Don't want to diminish what others here experience with their IDEs. Maybe I'm just having the "It works on my machine" syndrome but I would be interested about what OS and specific IDE OP ist using.
Wanted to try Niri or MangoWM for quite some time. But I'll wait till I replaced my Nvidia with AMD or Intel Arc. When I run into problems I'll report back. On YouTrack there are a few, but not overly many bugs and visual glitches related to Niri listed. No issue which resembles your specific problem. Would open an issue there or try to use another JVM for CLion. Good luck.