Delay adding SharedBounds / SharedElement

[riggaroo]

Executive Summary

Optimizing lookahead shared element modifiers and Compose runtime CompositionLocal usage resulted in dramatic frame timing improvements across both high-frequency scrolling and navigation transitions:

• Feed Scrolling: Median CPU frame time dropped by 52.9% (from 10.4 ms down to 4.9 ms), and P99 frame latency dropped by 64.8% (from 45.4 ms down to 16.0 ms), completely eliminating frame drops during scroll flings.
• Detail Navigation: CPU frame duration at P90 improved from 14.1 ms down to 9.5 ms, and P99 frame overrun improved from +9.9 ms (dropped frames) to -2.2 ms (zero dropped frames throughout the entire forward and reverse transition).

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1. Feed Scrolling Benchmark (scrollFeedFrameTiming)

Test Action: Rapid vertical flings down through the feed and back up.

Frame Duration (CPU Time per Frame)

Percentile	Baseline (Pre-Optimization)	Optimized (Post-Optimization)	Absolute Reduction	Relative Speedup
P50 (Median)	10.4 ms	4.9 ms	-5.5 ms	⚡ 2.12x faster (-52.9%)
P90	17.4 ms	8.6 ms	-8.8 ms	⚡ 2.02x faster (-50.6%)
P95	21.9 ms	10.6 ms	-11.3 ms	⚡ 2.07x faster (-51.6%)
P99 (Peak)	45.4 ms	16.0 ms	-29.4 ms	⚡ 2.84x faster (-64.8%)

Frame Overrun (Frame Deadline Margin @ 60Hz / 16.6ms)

Negative values represent headroom before missing a vsync deadline; positive values represent dropped frames.

Percentile	Baseline (Pre-Optimization)	Optimized (Post-Optimization)	Verdict
P50	-5.8 ms	-11.2 ms	+5.4 ms additional frame headroom
P90	+1.7 ms (Jank / Dropped)	-7.5 ms (Smooth)	Dropped frames eliminated
P95	+5.9 ms (Jank / Dropped)	-5.4 ms (Smooth)	Dropped frames eliminated
P99	+29.7 ms (Severe Jank)	+0.3 ms	Eliminated ~30 ms jank spikes

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2. Snack Detail Navigation Benchmark (navigateSnackDetailFrameTiming)

Test Action: Tap snack card (Forward shared transition) ➔ Snack Detail Screen ➔ Press Back (Reverse shared transition).

Frame Duration (CPU Time per Frame)

Percentile	Baseline (Pre-Optimization)	Optimized (Post-Optimization)	Absolute Reduction	Relative Speedup
P50 (Median)	8.0 ms	6.4 ms	-1.6 ms	⚡ 1.25x faster (-20.0%)
P90	14.1 ms	9.5 ms	-4.6 ms	⚡ 1.48x faster (-32.6%)
P95	16.1 ms	10.4 ms	-5.7 ms	⚡ 1.55x faster (-35.4%)
P99 (Peak)	22.3 ms	13.7 ms	-8.6 ms	⚡ 1.63x faster (-38.6%)

Frame Overrun (Frame Deadline Margin @ 60Hz / 16.6ms)

Percentile	Baseline (Pre-Optimization)	Optimized (Post-Optimization)	Verdict
P50	-3.3 ms	-9.9 ms	Smooth throughout
P90	+1.3 ms (Jank / Dropped)	-6.7 ms (Smooth)	Dropped frames eliminated
P95	+5.0 ms (Jank / Dropped)	-5.8 ms (Smooth)	Dropped frames eliminated
P99	+9.9 ms (Jank / Dropped)	-2.2 ms (Smooth)	Zero dropped transition frames

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3. Implemented Optimizations

1. On-Demand sharedBounds Attachment via Layout Rect Tracking

• Problem: Every card in off-screen horizontal LazyRow items unconditionally attached 5 Modifier.sharedBounds lookahead layout nodes (card bounds, background, image, title, tagline). During vertical scroll flings, Compose prefetchers executed expensive lookahead passes for off-screen cards (compose:lazy:prefetch:execute:urgent taking up to 16.1 ms per item).
• Fix: Added onLayoutRectChanged to cards and dynamically attached sharedBounds only when items enter the visible window (fractionVisibleInWindow() > 0f).

var isVisible by remember { mutableStateOf(false) }

JetsnackCard(
    modifier = modifier
        .onLayoutRectChanged { bounds ->
            val visible = bounds.fractionVisibleInWindow() > 0f
            if (visible != isVisible) {
                isVisible = visible
            }
        }
        .then(if (isVisible) cardSharedBoundsModifier else Modifier)
)

2. staticCompositionLocalOf for Navigation & Shared Transition Scopes

• Problem: LocalNavAnimatedVisibilityScope and LocalSharedTransitionScope were defined with dynamic compositionLocalOf. Reading .current across dozens of lazy card items registered active snapshot state observers in Compose's slot table.
• Fix: Converted both scopes to staticCompositionLocalOf, eliminating runtime observer tracking since scope references are invariant within a destination.

val LocalNavAnimatedVisibilityScope = staticCompositionLocalOf<AnimatedVisibilityScope?> { null }
val LocalSharedTransitionScope = staticCompositionLocalOf<SharedTransitionScope?> { null }

3. Conditional Bypass in JetsnackSurface

• Problem: JetsnackSurface unconditionally inserted a CompositionLocalProvider(LocalContentColor provides contentColor) subtree for every card, image container, and badge.
• Fix: Bypassed the provider allocation when contentColor already matches LocalContentColor.current.

if (contentColor != LocalContentColor.current) {
    CompositionLocalProvider(LocalContentColor provides contentColor, content = content)
} else {
    content()
}

4. Scope Reference Hoisting

• Problem: Calling LocalSharedTransitionScope.current and LocalNavAnimatedVisibilityScope.current inside every leaf item created redundant slot table lookups during rapid item recycling.
• Fix: Hoisted scope reads to the parent SnackCollection and passed them down as parameters to HighlightedSnacks / Snacks and child items.