I have been using Flet (basically multilingual binding for Flutter) to build GUIs the last year or so, and in general the experience has been very good.
However, I've recently started work on a project that has need for a large number of controls--on the order of 2000 controls visible at one time--and I'm running into Flet's limitations. All the Flet controls have animations, which creates a good default experience when there's a few of them, but when you're using 2000 of them, simply passing your mouse over an area with a bunch of controls causes a cascade of small animations and the renderer explodes. Impressively, it usually doesn't seem to cause any performance lag, but it seems like the way they avoid lag is just by dropping the animations half-rendered which causes the window to flash all sorts of broken half-rendered gobbledygook to the screen. My approach has been to turn off animations as much as possible, but there are enough controls visible at any one time that even just rendering the without animations is running into issues.
I'll probably just deal with it for my first version--the core functionality is about number crunching and the user base is used to using much worse UIs--but I'm looking at lower-level tooling that can still remain cross-platform. In this post I'm seeing that Pragtical is advertising that they're written with SDL and I'm seeing some similarities between a code editor and what I'm doing, so it seems like that might be the solution to my problem.