Fixed Toolbar: removing popup removes ui-page-header-fixed class

Fixes #6987
Fixes #6939
@arschmitz seems like this padding stuff as well as the next comment's giant class string are residual from before my attempts to fix this particular issue. Should I just assume that the entire functionality of this `_destroy` should be scrapped and just re-implement this entire method?  What I mean is I just wanted to address this Issue without breaking things that already existed without a full-on widget review