If you add it to core, can you replace/modify mw.notify while you're
at it. The 2 main problems with mw.notify are that you can't see it
and it doesn't float, both of which are addressed in PostEdit's
version.
Yeah, we actually discussed this when we first moved to make PostEdit permanent, and ultimately opted not to at that time. It would be good if you or anyone else interested could add their two cents to the relevant bug https://bugzilla.wikimedia.org/show_bug.cgi?id=40307