[css-display] Blockifications should establish a new formatting context

[Loirooriol]

It seems that in all cases in which an element is blockified, it also needs to establish a new formatting context.

So instead of specifying both transformations everywhere, it would be simpler if https://drafts.csswg.org/css-display-3/#transformations said that blockifying causes the element to establish a new formatting context.

Then, other specifications that require blockifications (like flexbox or grid) wouldn't need to explicitly require a new formatting context, because it would already be implied.

Additionally, a flow-root inner display type becomes flow when blockified. Then someone could think that the element stops being a BFC root. Explicitly saying that this is not the case (because the element establishes a new formatting context) would clear up the confusion.

[tabatkins]

Nope, grid items get blockified, but subgrids do not establish a new FC.

It's definitely usually the case (and could probably use a note in the spec that you can generally expect it), but it's not fundamental to blockification.

[tabatkins]

(The Grid 2 spec doesn't explicitly specify that subgrids don't form a new FC yet, but that's definitely what happens - their layouts intertwine with their parent, which is the definition of "not a new formatting context". We'll fix this in Grid 2 in a bit.)

[Loirooriol]

OK, I didn't think about subgrids, https://drafts.csswg.org/css-display-3/#valdef-display-grid will also need to be updated.

[Loirooriol]

@tabatkins Then, I think the spec should say this:

If an element with a flow inner display type is blockified, the generated block container establishes an independent formatting context.