itch.io

I use GameMaker, which obviously your first thought is like "wait, what??"
Don't use GameMaker. They have ZERO UI support. Everything was written 100% from scratch for Zenith Grabber. That includes even basic stuff like positioning, scaling for multiple resolutions, mouse interaction, drawing to the screen, all of it. 99% of the code is just getting the UI to work. They also don't let you do a lot of cool things with your code and I'm often really restricted in what I can do. In hindsight I wouldn't have used GameMaker if I had known the uses I wanted out of Zenith Grabber; it's just too far into development that I can't switch now.

I see you're pretty good with C# so if you do want to go down the from-scratch approach that I use (which is important if you want really nice and dynamic UI) then I recommend Raylib-cs. It's the same sorta deal, but with a language that isn't terrible.