Hey, just wanted to say I checked this out on my stream yesterday. UI seems very clean and polished so far! I'm sure there's a million things you want to add, but one thing I would really love is the ability to have fill layers with masks, so it's easy to change the entire material of certain aspects of something. That's my typical workflow with Substance: set the color, roughness, etc. for the whole layer then mask off the areas that should use those values.
I did run into a bug when I was trying to rectangle select with shift held to select multiple areas. It kept failing to select a certain area, then when it finally did, the fill tool failed to fill that area, like the visual didn't match with what was actually selected.
I also struggled to zoom in far enough, but later discovered it was possible to scale the model. Might be useful to allow more zoom by default.
I know sometimes it's useful to see somebody else using your stuff, so here's the Twitch VOD of my development stream. It'll only be around for a couple weeks:
https://www.twitch.tv/videos/1917332892 I start using Union Bytes Painter at around 2h23m