[css-view-transitions] Broken state management in "capture the old state"

[emilio]

https://drafts.csswg.org/css-view-transitions-1/#capture-old-state-algorithm has:

For each element of every element that is connected, and has a node document equal to document, in paint order:
[...]

• If usedTransitionNames contains transitionName, then return failure.
  [...]
• Set element's "captured in a view transition" to true.
• Append element to captureElements.

Note in particular that, if we take the failure path, any previous element that has the "captured in a view transition" bit to true doesn't end up in namedElements, which means that that bit isn't cleaned-up at all afterwards. That seems wrong? cc @khushalsagar @noamr

[emilio]

I think we need to run the:

For each element in captureElements:

• Set element’s captured in a view transition to false.

Always before returning.

[noamr]

I don't follow. What is the "failure path" in "capture the old state"?

[emilio]

The "then return failure"? E.g. consider we visit:

<div id="a" style="view-transition-name: --foo"></div>
<div id="b" style="view-transition-name: --foo"></div>

We visit #a, set a's "captured in a view transition" to true.

Then we visit #b, and realize it's a duplicate name, and return failure. #a still has "captured in a view transition" set and nothing cleans it up.

[noamr]

The "then return failure"? E.g. consider we visit:

In that case, failure is returned, and the caller will skip the view transition, which would also clear the flag here.

[emilio]

@noamr how? #a is not in namedElements (also it wouldn't be a new element anyways, even if it returned success)

[noamr]

Oh I see, you're right @emilio. Care to PR a fix for this? If not I can get to it soonish.

[emilio]

I can try. cc @nt1m (WebKit has this bug, leaving the captured flag set here)