fix: select option style(#4290)

[Huauauaa]

# PR

## PR Checklist

Please check if your PR fulfills the following requirements:

- [x] The commit message follows our [Commit Message Guidelines](https://github.com/opentiny/tiny-vue/blob/dev/CONTRIBUTING.md)
- [ ] Tests for the changes have been added (for bug fixes / features)
- [ ] Docs have been added / updated (for bug fixes / features)

## PR Type

What kind of change does this PR introduce?

<!-- Please check the one that applies to this PR using "x". -->

- [x] Bugfix
- [ ] Feature
- [ ] Code style update (formatting, local variables)
- [ ] Refactoring (no functional changes, no api changes)
- [ ] Build related changes
- [ ] CI related changes
- [ ] Documentation content changes
- [ ] Other... Please describe:

## What is the current behavior?

文档站点中，Select / DatePicker 等挂在 `body` 上的下拉面板打开后，滚动 `#doc-layout-scroller` 时面板会随 Popper 上移，遮挡顶部固定的组件介绍（`.docs-header`）。根因是弹层 `z-index`（PopupManager 从 2000 起）远高于介绍区域有效层高，且不宜简单抬高 header（会盖住 Dialog / DocSearch / Notify 等）。

Issue Number: #4290

## What is the new behavior?

文档内容区滚动时，通过向 `document.body` 派发 `mousedown` / `mouseup` 触发组件已有的 `v-clickoutside`，自动关闭 body 挂载的下拉浮层，避免遮挡顶部介绍；锚点 affix 逻辑仍按原 debounce 更新。不改动组件库弹层全局 z-index，不影响 Dialog / DocSearch / Notify。

## Does this PR introduce a breaking change?

- [ ] Yes
- [x] No

<!-- If this PR contains a breaking change, please describe the impact and migration path for existing applications below. -->

## Other information

- 仅改动文档站点：`examples/sites/src/views/components-doc/common.vue`
- 合成事件派发在 `document.body`（Element）上，避免派发到 `document` 时 touch-emulator 的 `closest` 触发 `Illegal invocation`
- 建议自测：打开 BaseSelect 下拉 → 滚动文档区 → 面板关闭且无控制台报错；打开 Dialog demo 确认遮罩/弹窗层级正常

[coderabbitai]

Walkthrough

The document layout scroll handler now closes body-mounted poppers on every scroll event. Anchor-affix computation remains debounced through a separate function.

Changes

Document layout scroll behavior

Layer / File(s)	Summary
Scroll handler split examples/sites/src/views/components-doc/common.vue	The scroll handler dispatches bubbling mousedown and mouseup events on document.body to close body-mounted poppers. Anchor-affix computation moved to updateAnchorAffixOnScroll and remains debounced at 100 ms.

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

Merge Risk: 🟡 Moderate · up to 2ec64

Scrolling the documentation layout may close unrelated open UI elements, rather than only select or date-picker poppers. Scope the close behavior before merge to avoid unexpected interaction loss.

Poem

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The patch grows clear beneath the moon,
Small changes hop in place,
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🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
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 0…
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.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title relates to the select component, but the changes mainly close body-mounted poppers during document scrolling and update anchor-affix behavior. The title is only partially accurate.

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 0 files. (1 skipped: 1 unsupported.)

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Actionable comments posted: 1

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Inline comments:
In `@examples/sites/src/views/components-doc/common.vue`:
- Around line 390-391: Update the synthetic mousedown and mouseup dispatches in
the clickoutside test flow to target only body-mounted poppers, using the
existing scoped close API or registry instead of bubbling document events
through every registered directive; preserve outside-click behavior for the
intended consumers.

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ℹ️ Review info

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• examples/sites/src/views/components-doc/common.vue

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