Should letterboxing take in account optional components' heights?

As noted by @henry , the issue with the language change notification bar on startup triggering letterboxing on about:torconnect, which prompted @pierov to defrost #32411 (closed), then duped in favor of #31064 (closed), is a more general problem affecting us not just on privileged pages but everywhere a notification box is shown.

While thinking about possible solutions (spoiler: probably there aren't, at least in a browser which changes the content's client height to show optional UI), I've noticed the find bar and the dev tools are special-cased by the letterboxing calculations, taking their heights in account as an offset to the content height.

Now I'm questioning whether these special cases are actually necessary, since all the three components are placed outside the browser container and already constrain its size.

If not (investigation needed), I'll gladly get rid of their additional complexity, which I suspect would help fix the hairy race condition behind #41434 (closed).