Browsers such as Chrome and Firefox apply an arbitrary scale to the
current font size if `normal` is used for `line-height`. Firefox uses
1.2 while Chrome uses 1.15. Let's go with the latter for now, it's
relatively easy to change if we ever want to go back on that decision.
This also requires updating the expectations for a lot of layout tests.
The upside of this is that it's a bit easier to compare our layout
results to other browsers', especially Chrome.
Sometimes fixed positioned boxes would extend the viewport's scrollable
overflow, which according to the spec should never happen. There are
some nuances to this, such as properly determining the fixed positioning
containing block for a fixed position box, but for now this prevents
some pages from being overly scrollable.
Fixes horizontal scrollability of https://tweakers.net.
