[css-view-transitions-1] Clarify implications of scrolling during a transition

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
45/100
Issue type
Documentation
Clarity
Mostly clear
Activity status
Stale
Tech stack
css
Domain
documentation

Research direction

Read the CSS View Transitions Level 1 text covering ::view-transition-group, its computed transform, and the UA view-transition-group animation keyframes. Confirm how document scrolling affects the initial and implicit final positions, then add a non-normative explanation of the observed scrolling behavior and verify the Bikeshed specification builds successfully.

Written by the indexing model from the issue text.

Description

css-view-transitions-1 Needs Edits

My understanding is that the computed value for 'transform' that is set for the ::view-transition-group selector includes the current scroll position of the Document (as required to place it in the current visual position if the view-transition is started when scrolled down).

This means that the -ua-view-transition-group-anim-transitionName 'from' keyframe includes the initial scroll position, and the (implicit) 'to' keyframe includes the current scroll position (and changes each frame if scrolling happens).

This has the visual effect that scrolling lags behind the movement of the scrollbar (and feels heavy), since the pseudo elements animate to the new scroll position. Obviously this is more pronounced when using a longer transition duration.

Assuming that this is the intended behaviour, it might be worth adding a non-normative explainer so that it's more obvious that this will happen.

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