Firefox has AppKit draw the menulist's capsule and chevron, but it sizes
the box and positions the label itself, using a fixed dropdown border
and the label's CSS margins. On macOS 27 (and probably 26), that left the
label closer to the edges and the chevron than in a native NSPopUpButton.
It also made the box 26.5px tall. Firefox assumes regular pop-up buttons
are 22px tall and only draws them unscaled in boxes up to 2px taller, so
it drew the control at 22px and scaled the image up, enlarging the
capsule and chevron.
Add padding to match native label insets, and trim the label's vertical
margins to keep the box at 24px, so Firefox draws the control unscaled.
The result measures the same as a native NSPopUpButton.
Firefox stopped inflating native controls on Tahoe (bug 1992898), but
still uses the old widget border sizes, so labels nearly touch the edges
of the new capsule-shaped buttons and run into menulist arrows.
Bug 2008041 made disabled, checked, hidden, collapsed, and selected
boolean attributes, so their value is empty and [disabled="true"] no
longer matches. Nothing sets any of them to "false", so matching on
presence alone is equivalent.
I think they're still slightly off center, but now they look exactly
like they do in Firefox. Dropdown labels in Apple's native macOS apps
are actually offset toward the top by a pixel or two.
Fixes#4086
