Block stretch sizing fixes

[nicoburns]

Objective

• Fix block nodes stretch-fitting when they shouldn't.

Context

• Block nodes were (unconditionally) stretch-fitting themselves. However block nodes only stretch-fit into available space if their parent is also a block node (it is really block containers that cause stretch-fitting, not block children).

Changes made

• Block nodes no longer stretch-fit into available space
• The root node (regardless of display mode) now does stretch-fit into available space (if it is definite). I think this is correct, but not 100% sure.

Future work

• Block nodes still unconditionally stretch-fit their children. This is usually what we want, but there are exceptions in HTML (for example table children of blocks do not stretch-fit) which we will likely want to support in future.

Feedback wanted

Does the root node being forced into taking up all available space (in the horizontal axis) when available space is definite actually make sense?

[TimJentzsch]

Are you able to define a test case for this which wouldn't pass for the old version and passes now?
I think that would be great to add to prevent regressions.

I'm not sure about the root node behavior... 🤔

[nicoburns]

I think this is definitely better than what we had before. We can iterate on the root layout as required.