fix(web): reserve sibling column width when resizing the right panel

[Lasdw6]

What Changed

The right panel's (diff/preview) resize clamp now reserves a minimum width for its sibling column (the chat, or the pull-request list) instead of only capping at 70% of window.innerWidth.

• getPreviewPanelMaxWidth takes the panel's flex-row width and clamps to rowWidth − 360px, keeping the viewport fraction as a secondary cap and flooring at the panel's own 360px minimum so a too-narrow row cannot invert the resize clamp.
• The row is measured with a shared measure callback (before first paint, then via ResizeObserver), so the clamp reacts to window and sidebar resizes and a persisted over-wide width cannot flash on mount. Measurement is skipped in modes that never apply the width (sheet, embedded, maximized).
• Unit tests cover the fraction cap, the row clamp, and both narrow-row edge cases.

+90/−10 in PreviewPanelShell.tsx and its test file.

Why

Fixes #6181.

The clamp was blind to the app sidebar (which sits outside the 70% math) and to the chat composer's minimum content width. Whenever 0.3 × viewport − sidebar fell below what the composer needs, the composer overflowed the chat panel before the clamp engaged.

That threshold is roughly a 1950px viewport, which is why the issue looked macOS-only: every MacBook — even fullscreen at 1512–1728 logical px — is below it, while typical Windows desktop monitors are above it. Reproduced on Windows 11 at non-maximized window widths and on macOS fullscreen; the drag now hard-stops leaving the sibling column usable on both.

UI Changes

Before/after screenshots and drag recordings are in the comment below (macOS fullscreen, matching the issue's environment).

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

Note

Reserve sibling column width when resizing the right preview panel

• Updates getPreviewPanelMaxWidth in PreviewPanelShell.tsx to accept an optional containerWidth and subtract a SIBLING_COLUMN_MIN_WIDTH (360px) reservation, so the preview panel cannot grow into the sibling column's space.
• Replaces the viewport-only useViewportClampedMaxWidth hook with useClampedMaxWidth, which tracks both viewport width and the parent flex-row container width via ResizeObserver and useLayoutEffect to avoid over-wide flashes on mount.
• Measurement is skipped entirely when not in inline non-maximized mode, avoiding unnecessary DOM observation.
• Behavioral Change: in inline non-maximized mode, the panel's max width is now clamped by available container space rather than viewport width alone; the panel will never shrink below its own 360px minimum.

^{Macroscope summarized f6ed169.}

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Note

Low Risk
UI-only resize clamp change in the preview panel shell; no auth, data, or security-sensitive logic.

Overview
Stops the inline preview/diff panel from squeezing the sibling chat (or PR list) column on narrower windows by clamping max width to row width − 360px, not just 70% of the viewport.

getPreviewPanelMaxWidth now takes an optional container width, keeps the viewport fraction as a secondary cap, and floors at the panel’s own 360px minimum so a too-narrow row cannot invert useResizableWidth’s clamp. The flex row is measured with useLayoutEffect + ResizeObserver (skipped outside inline non-maximized mode) so window and sidebar resizes re-clamp without an over-wide flash on mount.

^{Reviewed by Cursor Bugbot for commit f6ed169. Bugbot is set up for automated code reviews on this repo. Configure here.}

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[macroscopeapp]

Approvability

Verdict: Approved f6d8dff

Straightforward UI bug fix that adds container width measurement to prevent the preview panel from squeezing its sibling column below minimum width. Changes are self-contained to layout constraints with comprehensive test coverage.

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[Lasdw6]

Before

t3code.before.mov

After

t3code.after.mov