[css-view-transitions-2] How should pinch-zoom / visual viewport differences between states be handled?

[yoavweiss]

Note

🤖 This issue was filed by an AI agent (Claude, running in the Pi coding agent) on behalf of @yoavweiss, while migrating still-relevant issues out of WICG/view-transitions before that repository is archived.

This is a migration of an existing discussion, not a new proposal. The original author is credited below and the original thread is linked. @yoavweiss is accountable for it — please correct, retitle, or close if the migration is misjudged.

Migrated from: WICG/view-transitions#178 — originally filed by @jakearchibald on 2022-08-10. Most of that issue is now resolved; this migrates only the unresolved part.

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Already resolved (for context, not part of the ask)

The original issue covered two things, both since settled:

• Browser UI differences (scrollbars, URL bar, virtual keyboard) — resolved by the snapshot containing block, which is deliberately "consistent regardless of root scrollbars or interactive widgets". This is a better answer than the original proposal of padding the smaller root image with transparent pixels. Related: Does root capture scrollbars? WICG/view-transitions#133.
• Snapshot viewport size changing mid-transition — resolved via initial snapshot containing block size; a change causes the transition to skip. Discussion in [css-view-transitions-1] Handling snapshot viewport sizes changes during transitions #8045.

The remaining question: pinch-zoom

What should happen when the visual viewport differs between the old and new states, e.g. the user pinch-zoomed?

This is not covered by #8045: pinch-zoom changes the visual viewport without necessarily changing the snapshot containing block size, so the skip-on-resize behaviour doesn't trigger. Neither css-view-transitions-1 nor -2 mentions pinch-zoom or the visual viewport at all.

The original issue laid out three options, with @jakearchibald preferring the third:

• Abandon transition if zoom levels are different. Feels like this would drop too many transitions unexpectedly.
• Transition elements in visual viewport space. I think this would create some really weird transitions the developer wouldn't expect.
• Transition the visual viewport as part of the transition. I think this is my preferred option.

Filing narrowly so the question has a home; the two resolved halves need no further action.