fix: avoid selected item text color lagging behind thumb motion

[EmilyyyLiu]

背景

修复 ant-design/ant-design#37192

当选中项使用自定义深色背景 + 浅色文字时，切换项会导致文字颜色滞后于 thumb 动画——文字只有在动画结束后才变色。维护者已确认期望行为：「点击后就应该直接更新」。

根因

在 InternalSegmentedOption 中，item-selected class（同时承载背景色和文字色）受 thumbShow 闸门控制：

```js
[`${prefixCls}-item-selected`]: optionValue === rawValue && !thumbShow,
```

thumbShow 在 thumb 动画期间为 true，因此新选中项故意延迟挂载该 class，以避免出现双背景（新项自身的白色背景 + 移动中的 MotionThumb）。副作用是：文字色也被一起延迟了。

修复

将文字色拆分到独立的 item-selected-text class，让它立即跟随值，不受 thumbShow 控制：

• `src/index.tsx`：在选中项上新增 `item-selected-text` class（`optionValue === rawValue`，无 `thumbShow` 闸门）。
• `assets/index.less` / `assets/index.css`：新增 `&-selected-text { color: @text-color }`。保留 `&-selected` 原有的 `color` 以做向后兼容。

这样把文字色（立即）与背景色（延迟到动画结束）解耦，文字不再滞后于 thumb 动画。

测试

• 更新了快照。
• 将断言替换为针对 `item-selected-text` 的断言，并新增一条断言验证：当 thumb 动画进行中（`thumbShow` 隐藏 `item-selected`）时，新选中项仍持有 `item-selected-text`。

```
Tests: 31 passed
Snapshots: 13 passed
```

cc @afc163 @vagusX

🤖 Generated with Claude Code

[vercel]

Someone is attempting to deploy a commit to the afc163's projects Team on Vercel.

A member of the Team first needs to authorize it.

[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: 18c96c7d-fafb-4151-9805-a32c462ec610

📥 Commits

Reviewing files that changed from the base of the PR and between b59ce60 and c98bf95.

⛔ Files ignored due to path filters (1)

• tests/__snapshots__/index.test.tsx.snap is excluded by !**/*.snap

📒 Files selected for processing (3)

• assets/index.less
• src/index.tsx
• tests/index.test.tsx

Included review availability: Your plan includes up to 1 review per rolling hour; 0 remain after this review.

---

Walkthrough

Changes

分段项选中文本状态

Layer / File(s)	Summary
选中文本状态与样式 src/index.tsx, assets/index.less, tests/index.test.tsx	当前值对应的项目始终获得 rc-segmented-item-selected-text 类名。该类名使用 @text-color。测试验证 thumb 动画期间保留文本选中样式，且不添加 rc-segmented-item-selected。

Estimated code review effort: 1（简单）| ~5 分钟

Merge Risk: ⚪ Minimal · up to c98bf

This localized styling fix separates immediate text-color updates from the animated background behavior, with no actionable merge-blocking risk remaining after normal checks and review.

Suggested reviewers: afc163, qdyanbing

Poem

小兔轻点 Web3，
文字选中不迟疑。
thumb 滑过深色底，
白字稳稳守位置。
类名清晰，测试齐。

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	代码新增即时更新的选中文本样式类，并保留动画期间的 thumb 样式，符合问题 #37192 的修复目标。
Out of Scope Changes check	✅ Passed	修改仅涉及选中文本样式、组件类名和对应测试，均与问题 #37192 直接相关。
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	标题准确概括了修复选中项文本颜色滞后于 thumb 动画的问题，与主要变更一致且表达清晰。

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.39%. Comparing base (13dcd71) to head (c98bf95).
⚠️ Report is 17 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #342   +/-   ##
=======================================
  Coverage   99.38%   99.39%           
=======================================
  Files           2        2           
  Lines         163      164    +1     
  Branches       39       40    +1     
=======================================
+ Hits          162      163    +1     
  Misses          1        1           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.