Whole-plan coherence review (Plan 3a Layout + Plan 3b Motion) found that
Layout's keyframe gutter and Motion's Timing row independently derived an
element's start/duration and disagreed whenever an element had animations
but no explicit data-duration: Motion correctly inferred the range from the
element's GSAP tweens, while Layout's keyframe gutter fell back to a naive
`duration ?? 1`, so clicking a keyframe percentage in Layout could seek to a
different absolute time than what Motion's Timing row displayed.
Extract deriveElementTiming (propertyPanelFlatTimingDerivation.ts) as the
single shared basis both paths now consume: FlatTimingRow (Motion) and
PropertyPanelFlat's own elStart/elDuration (Layout's keyframe gutter and 3D
Transform block). PropertyPanelFlat now recomputes this basis itself from
its own element/gsapAnimations props instead of trusting the parent's naive
value, so PropertyPanel.tsx (and its legacy non-flat panel) is untouched.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
