fix(desktop): preserve workbar tab scrollbar behavior

[colaforniaw]

Summary

The app's global scrollbar default was a * rule in layer(components). A universal selector is an element-level declaration, so it competes by cascade layer — and components out-ranks astryx-components. That silently overrode the scrollbar-width: none Astryx's TabList sets on its overflow strip, re-showing a persistent bar under the Session Workbar tabs and nudging them off vertical center (a classic bar takes layout height).

Keep the universal scrollbar default, because scrollbar-width is not inherited and nested app scrollports must remain thin, but move the standard properties and the existing WebKit capsule recipe into the existing @layer base block in maka-tokens.css. The later Astryx and product component layers can then own exceptions on their elements: the TabList strip's none wins while ordinary nested scrollports retain the app-wide thin default. Keeping the WebKit rules preserves the established capsule thumb, transparent gutter, and hover treatment; this PR changes their cascade layer, not the app's scrollbar visual baseline.

Refs #2538

Verification

• npm run lint, npm run format:check, npm --workspace @maka/desktop run typecheck, npm --workspace @maka/desktop run check:architecture, and npm run check:asf-headers pass.
• The Storybook build and targeted workbar smoke pass. The story asserts both sides of the ownership contract: the TabList overflow strip resolves to scrollbar-width: none, while an ordinary nested review scrollport resolves to thin.
• Moving the universal rule back into layer(components) makes the TabList assertion fail (expected 'thin' to be 'none'), so the original regression cannot return silently.
• product-shell-official-appshell--tail-prefetches-history-until-the-band-is-full fails in the same full-smoke run on this branch and on the base commit (pre-existing, unrelated).

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: Maka — root-caused the regression, wrote the fix and the story assertion, and drafted this description.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No

[me2seeks]

PR 5204 Review

结论

CHANGES_REQUESTED
层机制修复本身正确且最小：把 universal scrollbar-width/scrollbar-color 从 layer(components)（base.css 经 styles.css:35 以 layer(components) 导入）移入 @layer base（maka-tokens.css:1228，cascade-layers.css:65 声明 base < astryx-components < components），Astryx TabList 的 scrollbar-width: none 因此重新胜出；story 测试同时断言两侧（strip=none、review panel=thin），契约完整。

发现

[P2] 同一 diff 里还删掉了整套 webkit 胶囊滚动条样式（33 行），但 PR 描述完全未提，这是全 app 的视觉变化

• 文件: apps/desktop/src/renderer/styles/base.css（删除的 *::-webkit-scrollbar* 5 条规则）
• 问题：删除的注释写明"Both reference systems converged on the same recipe: a slim pill inset in a transparent gutter"——即胶囊 thumb 是有意的设计基线。而 Desktop 渲染器是 Electron（Chromium），所有平台用户的滚动条都会从自定义胶囊回退到 UA 默认细条（scrollbar-color 在 Chromium 下仍着色，但 2px 内缩胶囊 + hover 加深消失）。这不是修 bug 所必需：TabList 复现条的问题是 scrollbar-width 的层冲突，把 webkit 规则一并移入 @layer base 就能同时保住两者；当前是"移了 width/color、删了 webkit"，新旧行为不一致。
• 建议（二选一，请在 PR 中说明决策）：
  1. 恢复 5 条 webkit 规则，与 scrollbar-width/scrollbar-color 一起放进 @layer base（保设计基线，改动同样最小）；或
  2. 确认胶囊样式是有意退役——那请更新 PR 描述（当前只说"move it into @layer base"和"remove the old :root workaround"，未提 webkit 规则删除），并确认 Storybook 视觉基线已更新。

[P3] 描述里"remove the old :root workaround so the default has one owner"指向的规则在本 diff 和当前树上都不存在

• 文件: PR 描述 / apps/desktop/src/renderer/styles/base.css
• 问题：全仓库 grep 不到任何 :root + scrollbar 的规则（diff 前后都没有）。这句话可能是相对更早 base 写的过时描述，或在描述另一个已被合入的改动。
• 建议：把这句改成实际发生的事（若 P2 选方案 2，正好并入 webkit 退役的说明）。

验证

• 层序：cascade-layers.css:65 @layer reset, theme, base, astryx-components, astryx-tokens, components; → base 低于 astryx-components/components，新默认让位成立。
• 例外规则全部保赢：prompt-rail.css:66（none）、workhub.css:253/257、composer.css:540、terminal.css:48、runtime-host.css:75 都在 layer(components) 或无层（无层恒胜层），不会被 base 默认覆盖。
• 修复面：base.css 经 styles.css:35 以 layer(components) 导入，正是原 bug 中 out-rank astryx-components 的来源；移入 base 后该冲突消除。
• story 测试：play 中 within/waitFor/expect 均来自已导入的 storybook/test（:22）；断言 strip 的 scrollbarWidth === 'none' 与 review panel 的 'thin'，双向覆盖。
• 无残留：renderer 下唯一幸存的 webkit 规则是 prompt-rail.css:67（width: 0，与本 PR 无关且保赢）。

[colaforniaw]

Addressed:

• Restored the WebKit capsule scrollbar styles.
• Moved them with the standard scrollbar rules into @layer base.
• Updated the PR description and removed the outdated :root wording.

Lint, format, typecheck, Storybook build, and the targeted smoke all pass. The story still verifies strip=none and review panel=thin.