fixup! BB 41919: Letterboxing, add temporarily visible web content-size indicator on window resizing.

TB 44214: Fix letterboxing status indicator for RTL.

TB 44214: Drop unnecessary CSS rules.

TB 44214: Use CSS logical positions for the status panel, rather than
"left" and "right".

TB 44214: Drop upstream's rules for placing content.

TB 44214: Drop letterboxing gradient.

TB 44214: Separate out the essential --letterboxing-width and
--letterboxing-height rules into their own .letterboxing block, to have
the property values set on.

TB 44214: Use CSS nesting. Part 1.

TB 44214: Use CSS nesting. Part 2.

TB 44214: Move the letterboxing classes one element up from tabpanels to
tabbox.

This is because we need to restyle the tabbox.

TB 44214: Use a CSS class to show/hide the letterboxing border, rather
than setting the border-radius in javascript.

TB 44214: Rename CSS variable from top-radius to radius-top. This is
closer to what upstream has done recently for tokens, where higher
specificity is appended.

TB 44214: Update letterboxing styling for ESR 140.