When searching in about:preferences, paneSearchResults is a virtual
category with no matching moz-page-nav-button. Because the nav is
declared `allownoselection="true"`, every button gets `tabindex=-1` when
that view is active, leaving no keyboard tab stop into the category
list.
Adds a standalone ghost moz-button "Back" affordance as the first child
of `#header-searchResults`. The header is always visible when a query is
active, so the button covers both the results and no-results cases.
Activating it clears the query and calls `history.back()`, which reuses
the previous pane's history entry so that `FocusHistory` can restore
focus to the search input. This results in the previous category pane
being restored when using the "Back" affordance.
Differential Revision: https://phabricator.services.mozilla.com/D306883
Tim Giles
3d8d2a47e7