[juliusmarminge]

RightPanelResizeHandle straddled the panel border (-left-1 w-2, z-20). In the Browser panel the webview is a fixed wrapper outside the panel shell at z-index 30, which holds the device-toolbar viewport rail, so the separator's in-panel half never took the pointer. Its only usable strip was the 4px inside the chat, at the end of the chat timeline's scrollbar lane, and the viewport rail started on the next pixel. The fix moves the separator's hit area inside the panel's own left edge (left-0 w-1) and raises it to z-40, above the webview layer. No ancestor creates a stacking context, so z-40 wins. The 1px hover/active line stays centered on the border (left-0 -translate-x-1/2, coarse pointer left-2), so nothing looks different. Verified on main and with the fix in the Electron desktop app over CDP, with the device toolbar on and the chat at its minimum width, hit-testing at y=400. Before: scrollbar lane 604-616 with the separator taking 613-616 inside it, and the rail 617-630. After: scrollbar 604-615, separator 617-620, rail 621-630. Drags after the fix: a 200px vertical drag on the scrollbar thumb at x=615 scrolled the chat (scrollTop 2503 to 1626). Dragging the separator resized the panel (484 to 424 and back). Dragging the rail resized the viewport (463 to 383). No new regex or parser.

Fixes #6273

Reproduction on main

Commit b68aedf (origin/main). Ran the Electron desktop app with T3CODE_DEV_SEED=0 headless-desktop vp run dev:desktop on Xvfb (1100x780 window) and drove it over CDP with Playwright. Steps: finished onboarding, added a git project, sent one 120-line user message and stopped the run, then expanded the message so the chat timeline scrolls. Opened the right panel, then Browser, loaded https://example.com, and turned on Preview menu > Show device toolbar (Responsive preset). Hit-testing with elementFromPoint at y=400, with the chat at its minimum width: chat scrollbar lane x 670-676; panel resize separator box 673-681, but it only takes the pointer at 673-676 because the panel content covers its right half; viewport left-edge rail takes the pointer at 677-690. Drags: pressing the visible scrollbar thumb at its right 3px (x=615 in the first layout, where the separator started at 613) showed cursor col-resize, and a 200px vertical drag did not scroll the chat (scrollTop stayed 5046). The same drag at x=611 scrolled it (5046 to 3481). A drag at x=615 resized the panel and a drag at x=686 resized the viewport, so every target works, but the separator's only usable strip lies inside the scrollbar lane, and the rail starts right where that strip ends. The rail only sits on the boundary while the responsive viewport fills the panel width; narrowing the viewport moves it inward.

Evidence

Both Electron apps: Linux/Xvfb, 1100×780, DPR 1, light theme, separate fresh databases and profiles. Same synthetic project, one UI-composed 120-line message, completed replay reply, and empty composer. Open Browser → example.com → Show device toolbar → Responsive 1024×768. Expand the message and start at scrollTop 2600. Perform the same scrollbar, panel, viewport, and emulated touch drags.

The right-edge thumb drag at x=615 leaves base at 2600, while head scrolls to 1017. The left-edge drag at x=611 scrolls both to 1017. At x=619, base changes the viewport width while the panel stays fixed; head moves the panel boundary 617→697 and retains viewport width 1024. The viewport rail works on both. Under coarse-pointer emulation, the hover line remains at x=616.5 on both; the touch drag moves the panel on head and reaches the viewport rail on base.

Before	After
b68aedfdb7	0316660717
Before: interaction recording	After: interaction recording

The completion and coarse-pointer pairs are pixel-identical. The boundary pair differs only within x=610–617. Both synthetic transcripts have SHA-256 169df396ed442f47071fafda5ff1dc866abe29a160121d5931994ed54019956e, one completed reply, and no interruption.

Reproduction instructions · Fixture script · UI driver · Normalization driver · Capture driver · Completion driver · Base measurements · Head measurements · Pixel comparison.

AI-assisted; verified with the tests and tools listed above.

Not shown:

• Physical touchscreen hardware was unavailable; the any-pointer:coarse CSS path and touch interaction were exercised in the real Electron app using CDP touch emulation.
• The original Stop interaction was not repeated in the final evidence; both sides instead awaited the same replay completion to eliminate the previously rejected run-state race.

Tests

vp run check:changed: pass (format, lint, knip 1/2 and 2/2, typecheck of 2 packages, react compiler, 1 nearby test file passed). Manual check in the Electron desktop app (headless-desktop vp run dev:desktop, T3CODE_DEV_SEED=0) driven by Playwright over CDP: elementFromPoint hit-tests and real mouse drags on the scrollbar, the separator and the viewport rail, before and after the change.

Not in this change

• On coarse pointers the separator still straddles the border (-left-2 w-4), reaching 8px into the chat. Touch devices usually have overlay scrollbars, and moving it fully into the panel would overlap the 10px viewport rail entirely.
• Dragging the separator to widen the panel does nothing while the chat is already at its minimum width. That is the existing width clamp, unchanged.

AI-assisted; verified with the tests and tools listed above.

[macroscopeapp]

Approvability

Verdict: Approved at 4cb0f3d

Macroscope's review found this PR approvable — The resize handle moves inside the panel boundary to avoid covering the chat scrollbar, with matching visual alignment. The change is limited to interaction styling and preserves existing resize handlers and coarse-pointer sizing.

Notes:

• All code in this push has already been reviewed. Approvability was decided on eligibility alone.

^{You can add or adjust custom eligibility rules. Learn more.}

[coderabbitai]

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Walkthrough

The preview resize handle changes from a two-unit-wide area offset to the left to a one-unit-wide area at the left edge. Its stacking order increases, and its separator moves to the left edge. Coarse-pointer overrides remain unchanged.

Changes: Preview resize handle

Layer / File(s)	Summary
Update handle hit area and separator apps/web/src/components/preview/RightPanelResizeHandle.tsx	The handle width changes from two units to one, its position moves to the left edge, and its z-index increases from 20 to 40. The separator also moves to the left edge. Coarse-pointer overrides remain unchanged.

Estimated code review effort: 2 (Simple) | ~5 minutes

Change: Bug fix · Severity of issue fixed: Low

Suggested reviewers: maria-rcks, macroscopeapp

Merge Risk: Minimal · previous review 03166

The change moves the panel resize hit area so it no longer covers the chat scrollbar. No actionable merge risk remains.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title names the main change: the panel resize handle no longer covers the chat scrollbar. The raw summary and objectives confirm the hit area moves inside the panel's left edge and rises to z-40, …
Linked Issues check		Issue #6273 asks for distinct, reliable pointer targets for the chat scrollbar, the side-panel resize handle, and the preview-content resize handle, with no overlap at the chat/preview boundary. The d…
Out of Scope Changes check		The whole-PR diff against the merge base touches one file, RightPanelResizeHandle.tsx, with a +2/-2 class change. Every change is on the separator's hit area, z-index, and visible-line position, which…
Description check		The description follows the repository template closely. It has a clear Problem section that describes the hit-area overlap with the chat scrollbar and the webview layer. The Change section explains t…

Full details: Title check

Explanation

The title names the main change: the panel resize handle no longer covers the chat scrollbar. The raw summary and objectives confirm the hit area moves inside the panel's left edge and rises to z-40, which addresses that overlap. The title uses the conventional commit form and is specific.

Full details: Linked Issues check

Explanation

Issue #6273 asks for distinct, reliable pointer targets for the chat scrollbar, the side-panel resize handle, and the preview-content resize handle, with no overlap at the chat/preview boundary. The diff changes only apps/web/src/components/preview/RightPanelResizeHandle.tsx. The separator wrapper moves from -left-1 w-2 z-20 to left-0 w-1 z-40, so its hit area sits inside the panel's left edge and above the webview layer. The visible 1px line keeps its centered position through -translate-x-1/2. Coarse-pointer classes (-left-2 w-4, span at left-2) produce the same geometry as before. The PR description reports Electron hit-test results: the scrollbar lane ends before the separator, and the separator ends before the viewport rail. This supports separate targets. The preview-content rail is not changed in code. Its separation depends on the reported layout measurements, which were not re-run here. The 'Evidence' section is still marked as pending capture, but the code change alone addresses the reported overlap.

Full details: Out of Scope Changes check

Explanation

The whole-PR diff against the merge base touches one file, RightPanelResizeHandle.tsx, with a +2/-2 class change. Every change is on the separator's hit area, z-index, and visible-line position, which are the subject of #6273. No unrelated files, refactors, or dependency changes appear in the reviewed range. The PR description's claims of no new regex or parser are consistent with the diff.

Full details: Description check

Explanation

The description follows the repository template closely. It has a clear Problem section that describes the hit-area overlap with the chat scrollbar and the webview layer. The Change section explains the move to left-0 w-1 and z-40, and why no stacking context interferes. The Scope section links #6273 as the bug issue. The Verification section gives specific hit-test coordinates, drag results, and before/after comparisons, plus screenshots and recordings. It also lists what was not checked, which includes physical touch hardware. The title line uses a conventional commit style in the PR objectives. The author notes AI assistance and the untested coarse-pointer hardware. The description is thorough and mostly complete. Minor gaps: the template asks for a conventional commit title, and the PR title is not in the provided context, so this cannot be confirmed. The description also includes an extended evidence section, which goes beyond what the template requires but does not conflict with it. The template asks for the model and harness used when an agent did the work. The description says AI-assisted but does not name the model or harness. The Tests section runs to a single line and lists only the combined check and one manual check. It does not repeat the template's requirement to state focused tests, which it does describe in the Verification section.

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