fix(ui): Don't count anchor corrections in ScrollablePositionedList

[renefloor]

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Description of the pull request

Screenshots / Videos

What was happening:

The performLayout loop has a budget of 10 cycles (_maxLayoutCycles = 10). Each non-converging iteration increments count. With our custom fitAnchor logic, the lifecycle during fast scrolling was:

1. N cycles consumed by sliver corrections — when fast-scrolling loads new messages, SliverList children get laid out with new sizes and may request scrollOffsetCorrection to keep the viewport stable.
2. +1 cycle consumed by the fitAnchor branch (count += 1 before continue) — our custom code that overrides the anchor when content fits in the viewport.
3. +1 cycle consumed when applyContentDimensions returns false (pixel position clamped).
4. = N+2 total. When N ≥ 8 (which can happen during a fast fling through many messages), that exceeds 10.
   The fix:

The fitAnchor pass is a deliberate one-time correction, not an oscillation. The upstream library's UnboundedRenderViewport doesn't have this pass at all — it was our addition to handle the edge case where a short list would appear centered instead of bottom-pinned. This pass must not consume a slot from the oscillation budget. Removing count += 1 gives back that cycle, meaning you now get the full 10 cycles for actual sliver oscillations before the assert fires.

Summary by CodeRabbit

• Bug Fixes
  • Fixed viewport layout behavior in scrollable positioned lists: one-time anchor adjustments no longer consume layout-cycle budget. This improves stability and smoothness during rapid scrolling, reducing the chance of layout cycle errors and ensuring more reliable re-layouts of message lists.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 1e33606f-a507-478d-b39c-80ee5fd8b14e

📥 Commits

Reviewing files that changed from the base of the PR and between 114bfa9 and 20e6b78.

📒 Files selected for processing (1)

• packages/stream_chat_flutter/CHANGELOG.md

✅ Files skipped from review due to trivial changes (1)

• packages/stream_chat_flutter/CHANGELOG.md

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📝 Walkthrough

Walkthrough

The viewport layout logic no longer consumes a layout-cycle slot when applying a one-time fit anchor correction that adjusts effectiveAnchor. This preserves the _maxLayoutCycles budget for subsequent sliver-correction attempts, preventing the anchor adjustment from interfering with re-layout iteration limits.

Changes

Fit Anchor Layout Cycle Budget

Layer / File(s)	Summary
Fit anchor correction without budget consumption packages/stream_chat_flutter/lib/scrollable_positioned_list/src/viewport.dart, packages/stream_chat_flutter/CHANGELOG.md	When applying the fit anchor adjustment to flush leading content to the axis-leading edge in UnboundedRenderViewport.performLayout, the layout-cycle counter is no longer incremented. A changelog entry describing the fix for a RenderViewport layout-cycle error was also added.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Suggested reviewers

• VelikovPetar

Poem

🐰 A viewport hops without a care,
When anchors fit just right in there—
One budget slot? Now it's kept free!
Re-layout cycles dance with glee. ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and specifically describes the main change: preventing anchor corrections from being counted in the layout cycle budget for ScrollablePositionedList.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/performLayout-loop-count

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.38%. Comparing base (386807d) to head (20e6b78).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2693      +/-   ##
==========================================
- Coverage   65.38%   65.38%   -0.01%     
==========================================
  Files         423      423              
  Lines       26665    26664       -1     
==========================================
- Hits        17436    17434       -2     
- Misses       9229     9230       +1     

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