cmux-next sidebar: headers keep their layout on hover

[teamleaderleo]

Summary

Hovering sidebar headers moved things around (cmux-b2's flicker sweep, findings 2 to 4). Per the stability rule, hover now only shows or hides controls in slots that are always there:

• Group header: the edit, add and member-count slots are reserved at rest, so the name no longer narrows and re-truncates when the pointer enters.
• Machine (Projects) header: the add button's slot is reserved whenever the section allows adding.
• Titled section header: the chevron stays mounted and fades instead of switching isHidden.

The workspace row's close and badge swap (finding 1) waits for #17520, which changes that row's trailing layout.

Testing

• SidebarHoverStabilityTests (committed red in fbb66c0 before the fix): a truncated group header and machine header keep their name frame when hovered; a titled section's chevron is never hidden, at alpha 0 at rest and 1 on hover.
• Fleet build of this branch, captured on a capture Mac (captures in a comment).
• check-no-godfiles passes locally. No new strings.

Changelog

• Fixed: cmux-next sidebar header names no longer shift or re-truncate when the pointer moves over them.

🤖 Generated with Claude Code

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Summary by cubic

Fixes sidebar header hover instability so header names no longer shift or re-truncate when the pointer enters.

• Group headers reserve their edit, add, and member-count slots at rest; machine and titled section headers reserve their add button slots, so names keep their width across hover.
• Titled section chevrons stay mounted and fade between alpha 0 and 1 instead of toggling isHidden, avoiding remount flicker.
• Adds SidebarHoverStabilityTests covering name-frame stability and chevron fading.
• The workspace row's close/badge swap on hover still waits on cmux-next sidebar: opt-in tab counts and a per-workspace tabs chevron #17520.

^{Written for commit 4acc822. Summary will update on new commits.}

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[github-actions]

Not verified: macOS jobs did not run: compile and app tests were skipped on 4acc822b55; ci-status is green without them.

CI passes on 4acc822b55 (run 37495272957 attempt 1).
macOS jobs did not run: compile and app tests were skipped: Fast static checks skipped (the macos job needs both to succeed). Re-run CI once they pass, before merging.

Written by scripts/ci/classify_failures.py (ci-failure-attribution.yml); signatures are its SIGNATURES table. A machine verdict is the runner's fault, not this PR's; yours means the failing file is one this PR changes, also red on main that main's latest full suite fails the same way.

[teamleaderleo]

Batch queue: green. Batch stack a850c9d6742b on feat-cmux-next 83058d8c627d with #17520 #17591. Fleet production build: job 26e3b6e270eabe154e4d271d (cmux-ci artifact 26e3b6e270eabe154e4d271d (job 26e3b6e270eabe154e4d271d, tag nb-local-20261006-163850-1)). heavy tier did not run on this branch (run); your PR's own checks are the tests. Validated this PR at 4acc822b5513.

Next: land it yourself with gh-merge-green on this receipt.

[teamleaderleo]

Group header hover, long name, narrow sidebar. Each strip is at rest, then hovered, then at rest again.

Before: the name is laid out at full width at rest and re-truncates when the hover controls appear, so the text moves under the pointer.

After: the header always reserves the control slots, so the name truncates the same way at rest and on hover.