Secondary toolbar buttons are toolbar buttons with some extra rules,
mainly to make them wider and have visible labels. However, this
similarity is currently not clearly reflected in the implementation
because the secondary toolbar buttons use a different CSS class,
`secondaryToolbarButton`, compared to the other toolbar buttons that
use the `toolbarButton` CSS class. This also causes some duplication
in the rules and requires extra care to keep the common bits for the
`secondaryToolbarButton` class in sync with the `toolbarButton` class.
Fortunately, now that we have a dedicated CSS scope for the secondary
toolbar, we can simplify this by giving all secondary toolbar buttons
the `toolbarButton` class and explicitly listing the required overrides
in the `#secondaryToolbar` scope instead. Doing so removes most of the
special-casing for secondary toolbar buttons while explicitly listing
the differences in a single place for a better overview. It also lays
the foundation for making all toolbar buttons respect the
`browser.uidensity` Firefox preference later by reducing differences.
Co-authored-by: Calixte Denizet <calixte.denizet@gmail.com>
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