[css-flexbox] Definite sizes section could use clarification

[cbiesinger]

https://drafts.csswg.org/css-flexbox/#definite-sizes

Upon re-reading this section, I have a few suggestions for it:

• Item 3 is only referring to flex items who are stretch-aligned, I believe. Also, instead of sayings "items ... are ... definite" it should say "cross-sizes of items" are definite.
• Item 1 is not necessary, I believe. It follows automatically from item 3 / stretch handling combined with the fact that flex lines in single-line flex containers are sized to the container. So it seems this could be turned into a note / optimization opportunity (because you can avoid an extra layout pass in this case)

cc @fantasai

[tabatkins]

It looks like fantasai's edit addresses your first bullet point.

I just pushed an edit handling your second; you're right, once we genericize the language in Item 3, it covers the Item 1 case entirely, so I just moved Item 1 to being a note.

[fantasai]

The second commit 6dd2edc is actually not editorial. We depend on the stretched size being definite when sizing the item in step 3, so we can't collapse these two points.

[fantasai]

[Two issues reported in this issue: one was accepted as editorial, the other rejected as invalid.]

[cbiesinger]

sounds good.