lol, of course. I spend all this time narrowing down which mod is causing this problem and if it's because of a conflict, and I could have just come here for my answer!

Anyway, the area he's talking about is shown here. For reference, I'm on a 1080p monitor:

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1397176396

The area within the green trees is uneditable: selection markers disappear (trees can't be placed, roads cannot end in that area, etc) within the outlined box; this box an area on the screen as if a UI object is obstructing it.

This uneditable area seems to only happen in the Map Editor, and does not appear in a normal game map or the Asset Editor. I have not tried within any other modes, and after 2 hours I'm frankly done testing the conflicts.

This error (for I believe it an error) exists without any other mods installed, and persists regardless of which mods are installed.