.libraryHeader and the alphabet index's .letterHeading copied their
z-index: 2 from tiles.css's own pre-existing sticky .searchBar - which
never showed the problem in practice, since a single slim input has
little room to actually overlap a scrolling tile. A tile's own
.tileOverlay action buttons are also z-index: 2 though, and an equal
z-index ties toward whichever renders later in the DOM - a tile, always
after the header. Wrapping the header into a much taller block gave a
scrolling tile far more room to pass underneath before fully clearing
it, which is what actually surfaced the tie. Raised both to z-index: 3,
matching shell/Shell.css's own mobileTopBar.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
