Previously, layout recursively performed these steps (roughly):
1. Compute own width
2. Compute own position
3. Layout in-flow children
4. Compute own height
5. Layout absolutely positioned descendants
However, step (2) was pretty inconsistent. Some things computed their
own position, others had their parent do it for them, etc.
To get closer to CSS spec language, and make things easier in general,
this patch reorganizes the algorithm into:
1. Compute own width & height
2. Compute width & height of in-flow managed descendants
3. Move in-flow managed descendants to their final position
4. Layout absolutely positioned descendants
Block layout is now driven by the containing block, which will iterate
the descendants it's responsible for. There are a lot of inefficient
patterns in this logic right now, but they can easily be replaced with
better iteration functions once we settle on a long-term architecture.
Since the ICB (LayoutDocument) is at (0, 0), it doesn't rely on a
containing block to move it into place.
This code is still evolving along with my understanding of CSS layout,
so it's likely that we'll reorganize this again sooner or later. :^)
The box tree and line boxes now all store a relative offset from their
containing block, instead of an absolute (document-relative) position.
This removes a huge pain point from the layout system which was having
to adjust offsets recursively when something moved. It also makes some
layout logic significantly simpler.
Every box can still find its absolute position by walking its chain
of containing blocks and accumulating the translation from the root.
This is currently what we do both for rendering and hit testing.
LayoutReplaced now has intrinsic width, height and ratio. Only some of
the values may be present. The layout algorithm takes the various
configurations into account per the CSS specification.
This is still pretty immature but at least we're moving forward. :^)
