[css-sizing] Should last remembered size be removed if the element is removed from the document?

[Loirooriol]

https://drafts.csswg.org/css-sizing-4/#last-remembered

At the time that ResizeObserver events are determined and delivered, if an element has a last remembered size but does not have contain-intrinsic-size: auto, remove its last remembered size.

Note: The last remembered size is state attached to the element, not any particular box generated by the element. So long as the element retains contain-intrinsic-size: auto, it will remember its last remembered size even across changes such as going to/from display: none.

Should the last remembered size survive the removal of the element from the document?

If not, should this be checked "at the time that ResizeObserver events are determined and delivered"? Or should it be removed immediately?

[css-meeting-bot]

The CSS Working Group just discussed Last Rememberd Size unclear when auto, and agreed to the following:

• RESOLVED: At ResizeObserver time, forget an element's last remembered size if it is currently out of the document

The full IRC log of that discussion

<fantasai> Subtopic: Last Rememberd Size unclear when auto
<fantasai> github: https://github.com//issues/7516
<fantasai> oriol: spec says that when we store last remembered size, this survives box destruction, e.g. if you add 'display: none', the element will keep storing this last remembered size
<fantasai> oriol: my question here is if the element is removed from the tree
<fantasai> oriol: and then later re-inserted
<fantasai> oriol: will it still have its last remembered size from the previous position in the tree?
<fantasai> oriol: should we remove the last remembered size?
<fantasai> TabAtkins: I suggest whatever is easiest to implement
<emilio> https://github.com//issues/7532
<emilio> github: https://github.com//issues/7532
<fantasai> ...
<fantasai> TabAtkins: Rossen made the point that moving the element is done by removing the item ...
<fantasai> TabAtkins: but if you're e.g. re-ordering a list, you might want to keep that info
<iank_> yeah there are lots of cases for moving things around, and it should keep the same last size.
<emilio> q+
<fantasai> TabAtkins: The common case for removing from document and re-inserting is moving it around
<fantasai> TabAtkins: and the common case of that is re-ordering within a list of similar items
<fantasai> TabAtkins: so my suggestion is to keep the last remembered size until normal things remove it
<Rossen_> ack emilio
<fantasai> emilio: You can move things around, given how specced right now
<ydaniv> Is removing from layout tree much different from being removed completely from the DOM?
<fantasai> emilio: that means that if you move it, by the time you ??, it will already have a box
<fantasai> emilio: when we fire ResizeObserver calllback, if you move an element, it will have a new box and a new position in the tree
<fantasai> emilio: do we want to clear it if it's not in the document or doesn't have a box, i.e. when you fire the ResizeObserver event
<fantasai> emilio: [missed]
<fantasai> emilio: Deciding at the time of ResizeObserver doesn't prevent the moving case from working
<fantasai> TabAtkins: if you remove it from the tree, hold onto it a bit, ResizeObserver fires, and then you don't have a remembered size at that point
<fantasai> TabAtkins: when the ResizeObserver timing happens (well-defined), if the element is outside the document, we clear the last remember size
<fantasai> TabAtkins: because we forget the size in some cases at ResizeObserver timing
<fantasai> Rossen_: what do we do for animations?
<fantasai> TabAtkins: they're tied to the box
<fantasai> TabAtkins: but these are tied specifically to the element itself
<fantasai> TabAtkins: is there an implementation concern?
<fantasai> emilio: no
<fantasai> TabAtkins: I could lean either way
<fantasai> Rossen_: Oriol, do you have an opinion
<fantasai> oriol: No strong opinion, just wondering about it
<fantasai> oriol: I suppose we can leave it to not do anything special
<fantasai> oriol: Styles might change if you move it, or the old size might be strange in the new position
<fantasai> oriol: but can also be fine
<fantasai> oriol: so no strong opinion
<ydaniv> q+
<fantasai> iank_: The reason not to tie it to the box is that we have a lot of features that will switch out of the box
<fantasai> iank_: e.g. ?? will be very common fro an element to have potentially different boxes associated
<TabAtkins> s/??/like when using CQ, it/
<fantasai> ydaniv: c-i-s is only interesteding if you have content-visibility and still out of view, right?
<fantasai> ydaniv: so by the time that the element is in view, it doesn't really play any part, right?
<fantasai> oriol: it can play a part if you first lay out the element normally and it can store its size, and then you add content-visilbity: hidden, the it can be in the screen and keep its contents, and it will use its last remembered size
<Rossen_> ack ydaniv
<fantasai> oriol: we resovled this last week
<fantasai> TabAtkins: that's the exact weirdo case this is designed for
<fantasai> Rossen_: Ok, regardless if it's weird or not, we need to make it more predictable
<fantasai> Rossen_: if we specify that it fires, the previous size
<fantasai> Rossen_: would there be any objections?
<fantasai> TabAtkins: I think I lean very slightly towards it forgetting its size
<fantasai> TabAtkins: if you're stashing away, not just doing a move
<flackr> q_
<flackr> q+
<fantasai> TabAtkins: it's much less clear that the new context you're putting it into would be similar
<flackr> q-
<fantasai> TabAtkins: so I lean towards forgetting if it's out of document
<fantasai> Rossen_: when you last checked this, did you check what implementations did?
<fantasai> oriol: ???
<fantasai> oriol: and in Gecko haven't landed implementation
<vmpstr> s/???/Chromium never seems to remove last remembered size/
<fantasai> flackr: For ?? scroller recycling element, I think we'd see this happen, and you'd want to forget the size, because you're changing the content before re-inserting
<flackr> s/??/virtual
<fantasai> Rossen_: so then the proposed resolution is that the element will forget its last size
<fantasai> TabAtkins: at normal ResizeObserver timing
<fantasai> vmpstr: then if removed and then re-aded, then it's ??????
<fantasai> TabAtkins: At ResizeObserver time, you also check if it is in the document
<fantasai> TabAtkins: if not, forget the size
<vmpstr> s/??????/keeps its size if it happened before the resize observer timing/
<TabAtkins> RESOLVED: At ResizeObserver time, forget an element's last remembered size if it is currently out of the document