[flexdinesh]

• Behavior owner and resulting change: app shell CSS (tokens.css, style.css, review.css) owns light-DOM scrollbars. Every scroller gets a thin 10px track with a 6px rounded thumb that appears while its pane is hovered or holds focus and darkens on hover/drag, matching Pierre's in-file tracks. Diff viewport, file tree, comments and Reviews column set scrollbar-gutter: stable, so content, tree rows and toolbar actions no longer shift when overflow toggles. Also use-file-navigation.ts: the hide-reviewed hand-off now pins the next file, the same way explicit navigation does.
• Contracts/invariants affected; local/remote implications: Pierre shadow DOM untouched; itemMetrics.paddingBottom and in-file horizontal measurement unchanged. scrollbar-color is set only for Firefox (@supports not selector(::-webkit-scrollbar)), because it inherits into Pierre's shadow roots and would disable Pierre's Chromium ::-webkit-scrollbar styling. --diff-viewport-inset is now constant at the track size. The hand-off pin fixes a race from feat: Alt+M toggles reviewed on selected file #127: when hide-reviewed handed selection to the next file without a pin, a scroll that landed before the shortened list laid out let followScroll reselect the top file. Under check-runner env (GOMAXPROCS=2) app.spec.ts:1044 failed 10/20 on main. No mode-specific behavior.
• Validation results; remaining risks: pre-push check:push 104/104. New browser test asserts a constant 10px gutter on the diff viewport and file tree with and without overflow, plus stable tree-row and toolbar edges; it fails with the gutters removed. Alt+M test 20/20 under check-runner env after the fix (main 10/20). Risks: panes reserve 10px even without overflow; macOS overlay-scrollbar users get a classic gutter in Chromium; scroll position is hidden at rest; Firefox picks its own thin width; the outer sidebar can still shift on very short viewports.
• Intentional architecture/design changes and rationale: DESIGN.md gains a Scrollbars rule (tokens, reveal, which panes get stable gutters, Pierre owns in-file tracks), and the diff gutter wording now says the scrollbar inset is constant.
• UI evidence: headless Chromium never repaints :hover scrollbar parts, even on a minimal page, so hover reveal was checked in a real browser (thumb shows on diff hover). Light/dark geometry is covered by the existing native-scrollbar spec. Keyboard focus reveal uses :focus-within. Pierre geometry, selection and virtualization are unchanged (existing specs pass).