Actually don't stopPropagation() mouse events

4fd5fcf8566cb46b4197c05005256fa847a714c0 lets us not .stopPropagation()
mouse events, but I didn't actually do so:
https://github.com/mathquill/mathquill/commit/4fd5fcf8566cb46b4197c05005256fa847a714c0#commitcomment-5517563