[css-view-transitions-1] Nothing prevents multiple CallUpdateCallback runnables from being put on the queue

[emilio]

https://drafts.csswg.org/css-view-transitions-1/#setup-view-transition-algorithm:

Queue a global task on the DOM manipulation task source, given transition’s relevant global object, to perform the following steps: ...

If that happens multiple times without the phase being advanced, you can call the update callback multiple times (or the relevant old capture).

cc @noamr @vmpstr

[noamr]

Can you elaborate on "If that happens multiple times"? I don't see how you can setup the same transition multiple times?

[emilio]

Setup happens on each rendering update, so if two rendering updates happen before the task runs (which can happen because vsync is more prioritary than other tasks, for example) you run the setup steps twice

[emilio]

In particular:

• Rendering update, phase is "setup", update callback is queued.
• Rendering update, phase is "setup", another update callback is queued.
• First update callback runs, appends to the pending update queue.
• Second one runs, appends to the pending update queue.
• Queue is flushed, callback is called twice for the same transition.

[emilio]

s/setup/pending-capture, sorry :)

[noamr]

In particular:

• Rendering update, phase is "setup", update callback is queued.

After this phase I believe rendering should be suppressed?

https://drafts.csswg.org/css-view-transitions-1/#ref-for-setup-view-transition%E2%91%A1:~:text=Set%20document%E2%80%99s%20rendering%20suppression%20for%20view%20transitions%20to%20true.

• Rendering update, phase is "setup", another update callback is queued.

This is invalid as per rendering supression.

[emilio]

Ah, blerg, https://drafts.csswg.org/css-view-transitions-1/#document-rendering-suppression-for-view-transitions doesn't mention anything about that, but it is referenced from HTML...

[emilio]

Thanks @noamr. Maybe it'd be worth pointing to the rendering section of the HTML spec from the rendering suppression flag, or something.