This issue tracks an open question about the behavior of Scoped View Transitions. Further context appears in the document Self-Participating Scopes.

Q: When we encounter a tag collision, which transition is skipped?

A "collision" means two transitions wanting to use the same element as a participant.

<div id="scope1">
  <div id="scope2">
    <div style="view-transition-name: foo"> PARTICIPANT </div>
  </div>
</div>
<script>
  scope1.startViewTransition(() => { ... });
  scope2.startViewTransition(() => { ... });
</script>

When a new transition wants to use an element that is already a participant in an existing transition, we have a few options:

• first wins - the new transition is skipped and the existing transition continues to run
• last wins - the existing transition is skipped and the new transition begins to run
• outermost wins - skip the transition whose scope is deeper in the DOM

@noamr pointed out that the outermost transition is probably more "important" visually. For example, it is more jarring to skip a whole-page transition than to skip a button transition. This argues in favor of the "outermost wins" strategy.

Note that this question only matters if a developer fails to set contain: view-transition on a scope. If every potential scope has contain: view-transition, then there cannot be any collisions.

cc @noamr @vmpstr @flackr @bramus @jakearchibald