When highlighting larger sections in the composer, the formatting bar
would scroll up and cover a previous post. This commit makes sure
the bar's offset will be limited to the top of the composer.

It should also make the code slightly more maintainable by getting the bar height
from the DOMrect and basing the offset on that height instead of hardcoding it.

Resolves: matrix-org#12359

Co-authored-by: Janne Mareike Koschinski <janne@kuschku.de>