Bug 1913322. That style requires the slot element to not have it's own box - previously that was already the case since <slot>
s have display: contents
by default, but with that style that specific slot was given display: flex
and thus have its own box. Thus, we remove it.
Which reminds me, by making it have display: contents
the tabs cannot overflow to scrollable box anymore. Instead they just overflow invisibly, which is no bueno.