Refactor the scrolling and scrollable area calculation on the window
object, to make it better match the specification. This has some mild
changes to behavior, but in general things work the same as they did
before. This is mainly preparation for properly handling viewport
propagation of the `overflow` property but seems to fix a few issues as
well.
There is one new failure in Layout 2020 regarding `position: sticky`,
but this isn't a big deal because there is no support for `position:
sticky` in Layout 2020 yet.
Co-authored-by: Rakhi Sharma <atbrakhi@igalia.com>
This is a simple code organization change with no behavior change with
the idea of making Layout 2020 easier to understand by new folks to the
project. The idea is that we will have a cleaner separation between the
different parts of layout ie one directory for the fragment tree and one
(currently multiple) directory for the box tree.
