Steps:

1. Click star to add to watchlist.
2. Click star again to remove from watchlist.

Expected:
Previous notifcation floats away in place.

Actual:
A glitch appears off to the top-left.

Other:

This affects all interactive or programmtic use of mw.notify(). The above is merely the simplest way to replicate it. Another way would be to e.g. run the following from the browser console:

mw.notify('Hello', { tag: 'something'});
mw.notify('Goodbye', { tag: 'something'});