osu-framework VS UnityCsReference

Related to your vision, you should definitely take a look at osu!framework. It's an open-source C# game engine, focused on 2D rendering and UIs. You can see the biggest example of it being used is, of course, osu! itself (osu!lazer, next iteration of osu!). It is so good that it has become my standard in terms of visual design and UI features.

An API I particularly like for this is how osu!framework does it.

Almost unrelated, but for game development (in C#), you might wanna look into the osu!framework, if at least just out of curiosity. It is a free and open-source game engine developed by peppy, the developer of osu!. You would also be able to develop on Linux (where programming tools really shine if you ask me) using VSCode and have neat things like visual tests and other stuff I haven't looked into.

it seems that despite using SDL, if you have "raw input" checked, it still uses the osuTK mouse input handler (https://github.com/ppy/osu-framework/blob/b97c26a684dc8ded5a349d24f8664a4f4b8c42a4/osu.Framework/Platform/DesktopGameHost.cs#L133, that's good)

My Favourite Graphics Engine at the moment is defnitly osu.Framework, you can find it here: https://github.com/ppy/osu-framework It can compile to .NET 5 and .NET 5 is cross platform now and the osu-framework makes smooth looking UIs with fancy transitions really really easy, it has some Audio Stuff in there aswell if you are looking to do something like that

I've checked their CS reference code on their Github, both here and here, but I want to dive a little deeper - specifically, I want to know what they're actually doing when determining validity of the Matrix4x4.

Yep, but beyond just decompiling it, the c# layer of code is on GitHub: https://github.com/Unity-Technologies/UnityCsReference

https://github.com/Unity-Technologies/UnityCsReference/blob/master/Runtime/Export/Scripting/UnityEngineObject.bindings.cs (after 2019.1)

The code where the method filtering for the Animation Event Inspector is done, is here (line 190). Is it possible to do something like this? That repository explicitly says:

No reason, you can look at the source of encapsulate

There isn't really a straight forward way to do this, even the QualitySettings editor uses Serialised Properties to get at the data. (Line 343 in QualitySettingsEditor)

This is awesome, thank you! I'm most curious about their Runtime

I think if you want to get into graphics programming you do want to work with OpenGL and similar things, because at the very least you need to understand it all (and decide what parts of engines to use and what to ignore when you get to whole games). It's also worth saying that while you can only publicly get the references in Unity you do get source access at the higher subscription tiers you'd use at a game studio.