Remember that time you were getting the hang of Blender, then you switched to XFCE and alt-mouse3 stopped working for Loop Select in Blender? Remember how annoying that was? Apparently alt-click is supposed to do stuff in XFCE even though it didn’t seem to really do anything in my case except mess up Blender. Well I finally figured out how to change this behavior in XFCE. In the main menu, Settings > Window Manager Tweaks > Accessibility, change the dropdown for “Key used to grab and move windows” to Super. Close the window and boom, Blender works again.
XFCE and Loop Select in Blender