feat(sidebar): reset width on double-click with viewport-based default

[chirivelli]

Double-clicking the resize rail restores the sidebar to 20% of the viewport width and persists the reset preference.

What Changed

Double-clicking the sidebar resize rail resets the width to 20% of the viewport. The reset is saved to localStorage.

Why

There was no quick way to get back to a default width after resizing. Double-click-to-reset is a familiar pattern and keeps the sidebar proportional across window sizes.

UI Changes

Double-click the resize handle between the sidebar and main content to snap the sidebar back to its default width. Video attached.

Screen.Recording.2026-08-12.at.2.05.12.PM.mov

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

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Note

Low Risk
UI-only sidebar sizing and localStorage preference changes; no auth, security, or data-handling impact.

Overview
Adds double-click-to-reset on the sidebar resize rail, snapping width back to a viewport-scaled default (~20%) and persisting that preference.

Replaces the fixed THREAD_SIDEBAR_DEFAULT_WIDTH with THREAD_SIDEBAR_DEFAULT_WIDTH_RATIO and resolveThreadSidebarDefaultWidth, so first-load and reset widths scale with the window (still clamped to min/max). AppSidebarLayout passes that live default into the sidebar’s resizable options.

^{Reviewed by Cursor Bugbot for commit 86b19d6. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Reset sidebar width on double-click using viewport-based default

• Replaces the fixed THREAD_SIDEBAR_DEFAULT_WIDTH constant with THREAD_SIDEBAR_DEFAULT_WIDTH_RATIO (0.2), so the default sidebar width scales with viewport size and is clamped between min and max bounds.
• Adds resolveThreadSidebarDefaultWidth(viewportWidth) in threadSidebarWidth.ts to compute this clamped default, and updates resolveInitialThreadSidebarWidth to use it as the fallback when no stored preference exists.
• Extends SidebarResizableOptions in sidebar.tsx with an optional defaultWidth prop and wires a double-click handler on SidebarRail that resets the sidebar to this default, persists it to localStorage, and calls onResize.
• Behavioral Change: first load or cleared-preference sidebar width is now viewport-relative (~20%) rather than a fixed pixel value.

^{Macroscope summarized 86b19d6.}

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[cursor]

Cursor Bugbot has reviewed your changes using high effort and found 1 potential issue.

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[macroscopeapp]

Approvability

Verdict: Approved 86b19d6

Self-contained UI enhancement adding double-click reset to sidebar and viewport-based default width. Changes are limited to sidebar components with simple clamped math, include tests, and only affect user-triggered interactions.

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