The LayoutFormattingContextEnabled header flag is not being respected in WebKitLegacy layout tests due to not being in the list of flags returned by TestOptions::supportedBoolWebPreferenceFeatures. Adding it causes a bunch of tests to fail, which is a bit unexpected.
Created attachment 422497 [details] Showcase
<rdar://problem/75395801>
@Alan - is this still needed?
dead codepath.