Responsive improvements to submenus in the toolbar.

Merges [27006] to the 3.8 branch.

 * Remove .ab-sub-wrapper rules in the 782px media query, as they're duplicates of the rules set on line 161.
 * Reposition the arrows for submenus at responsive sizes where the menus are taller.
 * Set the submenu position to static at smartphone sizes, so it's not positioned outside of its parent (and thus offscreen).

props undergroundnetwork, iammattthomas.
fixes #26720.

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