Guard geometry state writes against sub-pixel bounds jitter to prevent idle recomposition loops

[Aecasorg]

Fixes #488.

Problem

GeometryReader, onGeometryChange, and PresentationRoot write raw float Rect bounds from global-position callbacks into remembered state that gates their composed content. Bounds can differ from the previous layout pass by sub-pixel amounts (float rounding, scroll settle, in-progress animations, or content that sizes itself against the reported geometry), and mutableStateOf's structural equality only deduplicates bit-identical rects — so a sub-pixel delta closes a write → recompose → remeasure → write loop that recomposes continuously while the screen is idle. We hit this as a sustained idle-recomposition loop in our production Fuse app (skip-ui 1.57-era): a GeometryReader-centered empty state recomposed continuously at rest, and scoping the GeometryReader to a smaller subtree removed the trigger — which limits blast radius but leaves the unguarded write in place. Full mechanism citations in #488.

Change

Adds Rect.isApproximatelyEqual(to:) (all four edges within boundsEpsilonPx = 0.5f — half a device pixel, below anything visually meaningful) and guards the three gating state writes with it:

• GeometryReader.Render — rememberedGlobalFramePx
• onGeometryChangeErased — globalFramePx
• PresentationRoot — presentationBounds

Deliberately not changed: the shared onGloballyPositionedInRoot/InWindow helpers still deliver every callback. Deduplicating there would silently change semantics for callers whose derived values depend on more than the bounds — e.g. the safe-area edge probing in ComposeLayouts.swift recomputes edges from bounds plus the current safe area, and must not be suppressed when the safe area changes while bounds don't. Instead the helpers' doc comment now warns that callers gating content on remembered bounds should guard their writes.

The comparison intentionally avoids abs() — the transpiled skip.lib form is not comparable against a Kotlin Float (caught by the Android Kotlin compile).

Verification

Verified with CI-built APKs of a standalone counter-instrumented app (https://github.com/Aecasorg/skip-fuse-perf-repro), on an API 35 emulator, comparing stock skip-ui 1.57.0, stock 1.59.1, and 1.59.1 + this patch:

• Regression-free: four GeometryReader shapes (plain fillSize, inside ScrollView, refreshable ScrollView, proxy-derived text) plus unrelated-state and onChange control scenes behave identically with and without the patch — same body-evaluation counts, same settle points.
• Honest disclosure: the pure idle loop did not reproduce in those minimal synthetic shapes at either skip-ui version on the emulator — counters settle at 26–28 evaluations and stay flat. The production loop we observed evidently needs conditions the minimal scenes don't capture (device density, concurrent animation, richer layout). We're framing this change as a cheap, verified-safe guard against a demonstrated-in-production hazard rather than claiming a synthetic before/after.

Happy to adjust the epsilon, naming, or approach — including pushing the guard into the shared helpers instead if you'd rather own the call-site audit.

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[Aecasorg]

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