cmux-next: hover swaps an unread row's badge for its x in place

[teamleaderleo]

Summary

Hovering a workspace row with an unread badge swapped the badge for the close x at a different width. The name and the activity glyph beside it shifted on every hover. Now the x and the badge share one slot as wide as the wider of the two, so hover swaps them in place. Rows without a badge are unchanged: the x still appears on hover and narrows the name first, as the marquee expects.

Follows the hover stability rule from #17591: hover never moves known chrome.

Testing

• SidebarHoverStabilityTests.anUnreadRowsNameKeepsItsWidthOnHover covers a dot, a small count and a three-digit count. The test commit goes first, and this PR's cmux-next swift test lane records red on it and green on the fix.
• The fleet suite run on the red commit stopped early, on DebugMouseButtonTests (it needs a display), before reaching the sidebar suites.
• check-no-godfiles and check-concurrency pass.

Changelog

• Fixed: Hovering an unread workspace in the sidebar no longer shifts its name.

🤖 Generated with Claude Code

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Summary by cubic

Fixes the sidebar so hovering an unread workspace row swaps its badge for the close x without shifting the row's name or activity glyph. The badge and x now share a slot sized to the wider of the two, so the swap happens in place; rows without a badge keep the existing behavior where the x appears on hover and narrows the name first.

Testing

• Adds SidebarHoverStabilityTests.anUnreadRowsNameKeepsItsWidthOnHover, covering a dot, a small count, and a three-digit count.

^{Written for commit 843c615. 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 a9b27b042d; ci-status is green without them.

CI passes on a9b27b042d (run 37529482462 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]

Red and green on this PR's cmux-next swift test lane:

• Red, test commit a8e5bf0 (job): .dot title width went from 222 to 214 on hover, and .count(128) from 204 to 214.
• Green, fix 84dafc0 (merged as e8cba72, job): anUnreadRowsNameKeepsItsWidthOnHover passes with all 3 cases.

That lane was still red on WorkspaceRowMiddleClickTests and DebugMouseButtonTests, which #17624 fixes. a9b27b0 merges it in.

[teamleaderleo]

Batch queue: green. Batch stack 1a07d268cf62 on feat-cmux-next 423bd8a882c8 with #17584 #17602 #17609 #17611 #17625 #17630 #17633. Fleet production build: job 07b1c1720d6973c1d000950b (cmux-ci artifact 07b1c1720d6973c1d000950b (job 07b1c1720d6973c1d000950b, tag nb-local-20261006-192713-1)). heavy tier did not run on this branch (run); your PR's own checks are the tests. Validated this PR at a9b27b042d61.

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

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