Intent to unship: CSS "overflow: -moz-hidden-unscrollable" (legacy alias for "overflow: clip")

[Daniel Holbert]

As of yesterday (July 21 2025), I've removed support for the "overflow: -moz-hidden-unscrollable" CSS value. This keyword was a legacy value which at this point is just an alias of "overflow: clip" (which we continue to support, and which other browsers also support).  No other rendering engine ever supported "-moz-hidden-unscrollable"; it was a Mozilla-specific keyword that predated the standardization of "overflow: clip".

Bug to remove: https://bugzilla.mozilla.org/show_bug.cgi?id=1978450

This keyword has been off-by-default in Nightly for 3 years (as of https://bugzilla.mozilla.org/show_bug.cgi?id=1760734 ) with no reported problems, so it seems likely-safe to remove in general. Though: if needed, it can be reenabled by toggling about:config pref "layout.css.overflow-moz-hidden-unscrollable.enabled" to "true" (though eventually we'll remove that about:config pref, assuming the unshipping sticks.)