The .custom-select-dropdown used width: fit-content which, in Firefox,
is capped to the containing block width (60 px for the font-size input).
This caused two- and three-digit font-size values (e.g. 48, 120, 1000)
to be visually clipped in the dropdown list.
Switching to width: max-content lets the dropdown expand to fit its
content regardless of the containing block width, matching the behaviour
Chrome already exhibited with fit-content.
Fixes#11008