Delay adding SharedBounds / SharedElement

[riggaroo]

Delaying the addition of Modifier.sharedBounds / Modifier.sharedElement until composables are visible in the viewport using Modifier.onLayoutRectChanged reduces unnecessary lookahead layout/measure work and overhead during scrolling (especially on pre-fetched/offscreen items), leading to better frame timing and smoother scrolling.

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Test Setup & Methodology

• Benchmark Suite: JetsnackBenchmarks.kt
• Metric: AndroidX Macrobenchmark FrameTimingMetric
• Device: Pixel 9a AVD (API 36, ARM64)
• Compilation Mode: CompilationMode.DEFAULT (AOT / profileable release build)
• Startup Mode: StartupMode.COLD
• Iterations: 10 iterations per test run (~190 frames sampled per iteration across 3 full flings down and 3 flings up)

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Verified Side-by-Side Results

Metric	Baseline (Always Attached)	Optimized (onLayoutRectChanged)	Delta / Improvement
frameDurationCpuMs P50	32.4 ms	24.8 ms	-23.5% (faster)
frameDurationCpuMs P90	48.2 ms	34.9 ms	-27.6% (faster)
frameDurationCpuMs P95	56.1 ms	39.1 ms	-30.3% (faster)
frameDurationCpuMs P99	86.5 ms	56.8 ms	-34.3% (faster)
frameOverrunMs P50	17.1 ms	9.6 ms	-43.9% (less overrun)
frameOverrunMs P90	40.2 ms	25.8 ms	-35.8% (less overrun)
frameOverrunMs P95	53.8 ms	34.5 ms	-35.9% (less overrun)
frameOverrunMs P99	117.2 ms	79.4 ms	-32.3% (less overrun)

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Conclusion

The hypothesis is confirmed across multiple runs:

1. CPU Frame Duration: Decreased by 23.5% to 34.3% across all percentiles (P50 down from 32.4 ms to 24.8 ms, P99 down from 86.5 ms to 56.8 ms).
2. Frame Overrun (Jank): Decreased by 32.3% to 43.9%, significantly reducing jank during continuous multi-row flings.