fix: sync controlled scrollbar visibility

[nrps9909]

Summary

• synchronize the internal visibility state when boolean showScrollBar changes
• retain the existing delayed-hide behavior when the prop changes back to optional
• use the stable event helper for the timer callback and clean up replaced timers

Problem

showScrollBar currently initializes visible, but later boolean prop changes never update that state. A list rendered with showScrollBar={false} therefore stays hidden after changing to true, and the reverse transition can stay visible.

Testing

• the new exact-base regression failed on a573364: after false -> true, the vertical scrollbar still lacked its visible class
• the regression now covers false -> true -> false -> optional, including the existing three-second optional hide
• npm test -- tests/scroll.test.js --runInBand (38 passed)
• npm test -- --runInBand (8 suites, 281 tests passed)
• npm run tsc
• npm run lint (0 errors; 13 pre-existing hook warnings, down from 14 because the timer callback is now stable)
• npm run compile (ESM, CJS, and declarations)
• Prettier and git diff --check

Overlap audit

PR #381 touches the same source and test files, but only the non-primary mouse-button handlers and their separate regression. The visibility changes here are in independent hunks. The older #158 visibility proposal is a conflicting 2021 branch for adding an always-visible API; the current public showScrollBar API already exists and this PR fixes its controlled rerender behavior. Broad conflicting PR #199 does not cover prop synchronization.

AI assistance disclosure: Codex was used to trace the controlled-prop lifecycle, implement and validate the exact-base regression and timer behavior, audit open PR overlap, and draft this PR. All stated behavior and test results were verified locally.

Summary by CodeRabbit

• Bug 修复

  • 修复滚动条显示状态在不同配置间切换时未及时同步的问题。
  • 优化滚动条自动隐藏逻辑，确保可选模式下按预期延迟隐藏。

• 测试

  • 新增滚动条可见性切换及自动隐藏场景的验证。

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 07a0cb64-7354-4696-9b05-4f9ab2b225f6

📥 Commits

Reviewing files that changed from the base of the PR and between a573364 and 407ea2c.

📒 Files selected for processing (2)

• src/ScrollBar.tsx
• tests/scroll.test.js

Included review availability: Your plan provides up to 4 included reviews per hour; 2 remain after this review.

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Walkthrough

本次变更更新 ScrollBar 的可见性同步逻辑，并新增测试覆盖 showScrollBar 的状态切换和自动隐藏行为。

Changes

滚动条可见性

Layer / File(s)	Summary
可见性控制逻辑 src/ScrollBar.tsx	showScrollBar 为布尔值时直接设置滚动条可见性。其他情况继续使用 3000ms 延迟隐藏逻辑。effect 依赖项同步更新。
可见性状态测试 tests/scroll.test.js	测试覆盖 showScrollBar 从 false 切换为 true、false 和 'optional'，并验证延迟隐藏。

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 407ea

This change synchronizes scrollbar visibility when the controlling prop changes while preserving delayed hiding for optional mode. The behavior is covered by regression tests and no actionable merge-blocking risk remains after normal checks and review.

Suggested reviewers: zombiej, afc163, aojunhao123

Poem

小兔轻点滚动条，
真与假状态排成行。
optional 等三千毫秒，
月光下悄悄隐藏忙。
测试挥挥小手帕，
可见性同步闪亮亮。

🚥 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	标题准确概括了本次变更的主要目的，即修复受控滚动条可见性的同步问题。标题简洁且具体。
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 2…
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.

Full details: Docstring Coverage

Explanation

No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 2 files.

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests

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