Intellij - what is this strange selection, and how do I turn it off?

I think you have enabled 'column selection'.

Option can be toggled via these methods:

Menu Bar -> Edit -> Column selection mode

Document Body -> Right Click -> Column selection mode

Keyboard Shortcut -> ALT + SHIFT + INSERT

The column select mode is enabled, use Alt+Shift+Insert to enable or disable this mode.

Shift-command-8 to toggle it back and forth on OSX.