My guess is that most of API is generally there, but the issues i mentioned elsewhere in this post with the region stuff cause the glitches. This looks like a simple thing to fix (i've implemented regions at the past in an experimental winsys i was writing on Linux [0] so i have a general idea of how they work) for someone with knowledge about ReactOS' graphics stack, so i suppose there are reasons it hasn't been fixed yet (maybe it relies on driver support and they plan to change the driver model? Just an assumption).

[0] https://i.imgur.com/v6NaqNW.png