[css-display] Becoming a formatting context without display types

[Loirooriol]

Becoming a formatting context is defined as follows:

If the element already establishes a formatting context of any kind, this condition is satisfied. Otherwise, it is made to establish a BFC by changing its inner display type to flow-root.

But what happens if an element that does not have inner nor outer display types (i.e. it has display: none or display: contents) becomes a formatting context? Such an element won't establish a formatting context, so its inner display type should be changed to flow-root, but it doesn't have an inner display type!

I think "this condition is satisfied" should also apply in that case.

[SelenIT]

I suppose that becoming a formatting context implies creating a box. Maybe it's better to add a Note to the definition of these display values that they never establish a formatting context?

But it's an interesting case what happens if an element with display:contents gets contain:layout or some other property that usually causes an element to establish a FC. Should that property be ignored at all, should it apply to anonymous wrapper box, or something else should happen?

[frivoal]

https://drafts.csswg.org/css-contain/#containment-layout :

If the element does not generate a principal box (as is the case with display: contents or display: none) [...] layout containment has no effect.

In other word, "becoming a formatting context" is not invoked by the contain property on display: contents elements. So from the point of view of that spec at least, it does not matter what the definition of "becoming a formatting context" is for that case.

[Loirooriol]

And the same for column-span, which only applies to in-flow block-level elements.

But the definition of "becoming a formatting context" should not rely on not being invoked with display: none or display: contents.

[frivoal]

Sure. I am just saying that form the point of view of these specs, it doesn't matter what we define it to be.

[tabatkins]

Boxes generate formatting contexts, not elements. I've at least patched that section of the "becoming a formatting context" definition.

The question is thus moot - none or contents prevents the element from generating a box at all, so there's no box to create a formatting context in the first place.

[Loirooriol]

Well, before 71cfe56, it were elements what could "become a formatting context". Changing it to boxes fixes this issue, but now "becoming a formatting context" is not well-defined, because boxes don't have display types (#1480). If you plan "become a formatting context" to affect computed display, it needs to happen on the element, otherwise there could be no display value which generates the resulting boxes.

[tabatkins]

Yes, the section as a whole needs to be rewritten - it's currently marked for discussion at the F2F next month.