Machine is 2020 MBP 13" i7 32gb. Drag the window and you'll see white flash on the background. This complaint is pedantic for sure, just as a note. I was having issues with dragging the separators before but these don't seem to be reproducible currently, so perhaps it was specific to my machine.
Another note is that typing in Notable pushes my CPU to about 10%+ usage. It doesn't do this with Notes. Also the smoothness of typing in native Notes vs this app is quite apparently, everything seems more fluid on the native app (it seems like Notable is, at times, struggling to keep up with rendering).
Again I'll note I'm very pedantic about the native feel of apps and I very much dislike electron / non-native type apps, so take my criticisms with appropriate amounts of salt. Nice job on the app!