perf(framework): drop redundant O(n) child-order copy in reconcile

[danReynolds]

Closes out the §2 retained-tree-tax investigation of the perf pass. Verdict: confirmatory — the retained-tree machinery is lean, and this lands the one genuine redundancy the investigation surfaced.

The change

MultiChildRenderObjectElement._syncChildRenderObjects pre-checked child order via owner.children, but that getter is List.unmodifiable(_children) — a full copy of the child list — and the check duplicated the identical hasSameRenderChildrenInOrder guard that replaceAllChildren already runs against its internal list with no copy. So every multi-child element paid a redundant O(children) copy + duplicate scan on every rebuild. Now it calls replaceAllChildren directly (which no-ops unchanged order itself) and the dead _sameRenderObjectOrder helper is gone (net −5 LOC).

§2 investigation (recorded in perf-pass-findings.md)

Isolating the steady-state no-op-rebuild tax from one-time mount:

tree	idle render	no-op rebuild	reconcile tax	tax/node
45	34µs	45µs	11µs	0.24
169	38µs	56µs	18µs	0.11
657	103µs	142µs	40µs	0.06
1465	213µs	300µs	85µs	0.06

The reconcile tax is sublinear per node and tiny (85µs even at 1465 nodes ≈ 0.5% of a 60fps budget). The aggregate profiler's replaceAllChildren dominance (27× the next symbol) is mount, not steady-state. The three big levers — layout-skip (clean subtrees), opt-in paint-memoization (RepaintBoundary), lean reconcile — are all present and correct. The earlier speculative CellConstraints-pooling target was mis-aimed (idle/paint-only frames run 0 layouts). This change is measured ~10% off the reconcile tax at mid sizes — a hygiene win, not a workload mover.

_RenderListView parity guard (not a regression fix)

A /code-review flagged that dropping the element-side pre-check would make the eager ListView re-layout on same-order rebuilds — _RenderListView.replaceAllChildren was the lone RenderObjectWithChildren missing the hasSameRenderChildrenInOrder guard. On verification this is refuted as an observable regression: _ListViewBody.updateRenderObject marks needs-layout unconditionally every rebuild (to re-read mutable controller state), so the eager ListView relayouts regardless — measured performedCount identical (7) with and without the guard. The guard is added anyway for parity with the other four implementations and to stay correct if that unconditional mark ever becomes conditional; the comment documents the masking. (The bogus performedCount == 0 regression test was dropped — it can never hold for the eager ListView.)

Verification

• dart analyze: clean.
• 1042 widget + rendering tests green — incl. the "same-ordered children are a layout no-op" / "reordered children still invalidate layout" invariants and the full list_view suite.
• SB.12 childListNoOpFrameUs ≈223→182µs median (noisy, 20 samples).

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR removes a redundant child-order pre-check in MultiChildRenderObjectElement._syncChildRenderObjects that forced an O(n) copy via RenderObjectWithChildren.children (List.unmodifiable(...)) and duplicated the order-guard already performed inside each replaceAllChildren implementation. It also adds the missing in-render-object order guard to _RenderListView.replaceAllChildren for consistency with the other RenderObjectWithChildren implementations, and updates the perf investigation notes accordingly.

Changes:

• Remove element-level child-order comparison in _syncChildRenderObjects and call replaceAllChildren directly (letting render objects no-op unchanged order without copying).
• Add hasSameRenderChildrenInOrder early-return guard to _RenderListView.replaceAllChildren.
• Update perf-pass-findings.md with the §2 investigation results and the identified redundancy/fix.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
packages/fleury/lib/src/widgets/framework.dart	Drops redundant element-side child-order pre-check that incurred an O(children) list copy and duplicated render-object guards.
packages/fleury/lib/src/widgets/list_view.dart	Adds missing same-order guard to _RenderListView.replaceAllChildren for parity with other multi-child render objects.
docs/implementation/perf-pass-findings.md	Documents the retained-tree-tax investigation outcome and the specific redundancy removed.

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