This patch just adds the new root paintable and updates the tests
expectations. The next patch will move painting logic from the layout
viewport to the paint viewport.
This patch does three things:
- Factors out the code that determines whether a box will create a new
formatting context for its children (and which type of context)
- Uses that code to mark all formatting context roots in layout tree
dumps. This makes it much easier to follow along with layout since
you can now see exactly where control is transferred to a new
formatting context.
- Rebaselines all existing layout tests, since the output format has
changed slightly.
When sizing a flex container with flex-direction:column under a
max-content height constraint, we were incorrectly truncating the
infinite available height to 0 when collecting flex items into lines.
This caused us to put every flex item in its own flex line, which is the
complete opposite of what we want during max-content intrinsic sizing,
as the layout would grow wide but not tall.
