In the "what fresh hell is this?" department, a new change in the last few days or so has made it so that after you edit a post (full desktop site), make a change*, and click Save, attempting to navigate away from the page results in a spurious "Leave Page? Changes you made may not be saved." dialog, which can mislead you into thinking you perhaps didn't save changes, when in fact you did. So apparently some state isn't being saved correctly.

*this is relevant; if you don't make any changes then I think you don't get the dialog.