ThickBox z-index conflicts with admin menu and bar

[shaunandrews]

The Thickbox modal, which is used (at least) to display theme update details is overlapped by the wp-admin menu and bar:

[nacin]

We should probably try to restore the admin menu to whatever z-index it was in 3.7. I'm guessing it changed.? Curious, though, how this didn't affect the customizer or media modal.

[shaunandrews]

related to #26280

[shaunandrews]

26442.diff fixes this by dropping the z-index of the sticky-menu from 9999 to 99, which lets the Thickbox (and its z-index: 100;) remain overtop.

[ryelle]

26442.2.diff​ also addresses the z-index issue in this video: ​https://cloudup.com/cpwFnV0zFIo, where the thickbox loaded behind the theme modal on smaller screens. I've adjusted the z-indices of the theme modal & thickbox: the theme modal's z-index is now slightly more sane, and thickbox has been increased.

[nacin]

In 26701:

Fix z-indexes as it relates to thickbox, the theme modal, and admin menu.

props ryelle, shaunandrews.
fixes #26442.

[nacin]

In 27833:

Restore the original z-index of the sticky admin menu, as first set in [26442].

The inline comment is now correct again. This z-index was changed in [26701], but those circumstances no longer apply after [27532].

props mordauk.
see #26442, #26952.
fixes #26567.