Unable to Shift or Control select in Map Editor

There's about a 50% chance that I'm actually able to Shift or Control select a lot of objects in the map editor. Sometimes, it just doesn't do anything at all. When this happens, I'm also unable to highlight an object and Alt+Click it into the map editor. Holding alt simply allows me to drags the view around when I move my mouse.

Does anyone else get this issue? My hunch is that for some reason, it's detecting that my mouse left click is always being held down when it's very clearly not...

Unable to Shift or Control select in Map Editor