I was wondering why I couldn't middle click on enemies. When Drag Scroll is disabled I can. When Letterbox is disabled I also can.
Turns out the button used for drag scroll seems to offset all clicks about one square to the left on the battlescape (even for left mouse/moving). Is it only me or everyone?
More info:
Version: Extended+-3.10a-632549f-2018-05-09-win32
Letterboxed: on (this seems to fix it when off)
Battlescape Scale: 2x
Windowed 1776x1000
Grab mouse: off (doesn't seem to matter)
EDIT: switching to 1/3 display also seems to fix the offset