[csswg-drafts] [css-anchor-position-1] Interaction between trying position options and CSS transitions (#13048)

tuankiet65 has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-anchor-position-1] Interaction between trying position options and CSS transitions ==
When [determining the position fallback styles](https://drafts.csswg.org/css-anchor-position-1/#determine-position-fallback-styles) for an element, it applies each position option on top of the base style to create a new style, then apply the style and see if it doesn't overflow or not. I have concerns that "apply the style" may lead to weird interactions with CSS transitions, since CSS transitions are automatically started on style change.

Some scenarios that I could think of:
1. If a property that could affect layout transitions, should position fallback be applied throughout the transition, or only after it ends? Say `width` is transitioning from `100px` to `200px`, and `150px` and above overflows the current position. Should we try position options when the transition hits `width: 150px`, or should it wait until the transition ends before trying?
2. When trying position options, which implies applying a new style, should transitions be started between style changes? Say an element has `position-try: flip-block flip-inline`. If we're changing from the base style to `flip-block`, or from `flip-block` to `flip-inline`, should this trigger transitions?
3. (kinda 1 + 2): If a property is transitioning, and it triggers position fallback, and it applies a new style, should transition be triggered?

The spec only says this, which I don't think says a lot:
> These modified styles are applied to the element via [interleaving](https://drafts.csswg.org/css-anchor-position-1/#style--layout-interleave), so they affect [computed values](https://drafts.csswg.org/css-cascade-5/#computed-value) (and can trigger transitions/etc) even though they depend on layout and [used values](https://drafts.csswg.org/css-cascade-5/#used-value).


Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/13048 using your GitHub account


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Received on Friday, 31 October 2025 21:52:09 UTC