refactor(desktop): derive the tab hover close button from the close verb

[ethernet8023]

What does this PR do?

The hover close button on a pane tab was wired separately from the close
gestures that do the same job. PaneTab gated the button on two independent
inputs — the onClose verb, and a showCloseButton prop that TreeGroup fed
from a showCloseButton flag on the pane contribution — while middle-click and
⌘-click read only onClose. A tab could therefore close on a pointer gesture
and show no control for it.

The flag had no user that hideOnly did not already cover. Both setters also
set hideOnly: true, which removes every close gesture:

• the sessions pane (apps/desktop/src/app/contrib/controller.tsx),
• the Bots pane (apps/desktop/src/plugins/hermes-bots/plugin.js).

The history explains why. #89551 added showCloseButton as an ✕-suppression
fix. #89572 cherry-picked that commit as its base, then built hideOnly on top
and superseded it. The flag stayed behind as a vestigial rung, not as live
design, so this PR deletes it instead of teaching it to track the gestures.

onClose alone now decides both shapes. A tab that closes shows the button. A
tab without the verb shows nothing. To make a tab uncloseable, give it no close
verb. hideOnly and uncloseable keep their meaning: they gate the verb, and
both shapes follow the verb together.

Related Issue

No issue. This is follow-up cleanup on #89572, which introduced hideOnly and
left the superseded flag in the tree.

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• apps/desktop/src/components/ui/pane-tab.tsx: delete the showCloseButton
  prop. The render condition becomes {onClose && !vertical && …}. The
  onClose doc comment states that the chip and the pointer gestures are one
  affordance.
• apps/desktop/src/components/pane-shell/tree/renderer/track-model.ts: delete
  showCloseButton from PaneChrome. The hideOnly comment now says that it
  removes the close verb, and that the ✕ follows the verb.
• apps/desktop/src/components/pane-shell/tree/renderer/tree-group.tsx: delete
  the showCloseButton={chrome.showCloseButton !== false} callsite.
• apps/desktop/src/app/contrib/controller.tsx: drop the flag from the
  sessions pane data. Its hideOnly: true already removes every gesture.
• apps/desktop/src/plugins/hermes-bots/plugin.js: the same for the Bots pane.
• apps/desktop/src/components/pane-shell/tree/renderer/tab-close-affordance.test.tsx:
  new contract test (see below). The DialogContent / SheetContent prop of
  the same name is untouched: it is a different prop, it has no close verb to
  derive from, and updates-overlay.tsx changes it while the dialog is open.
• apps/desktop/src/components/ui/pane-tab.test.tsx: move the "can hide the
  hover ✕" case off the deleted prop. It now asserts that a closeable tab shows
  the button and honors middle-click.

How to Test

Behavior is identical before and after, so the proof is the mutation test:

1. cd apps/desktop && npx vitest run src/components/pane-shell src/components/ui/pane-tab.test.tsx
   — 154 tests pass.
2. Break the derivation by hand: change the PaneTab render condition to
   {false && onClose && !vertical && (, which hides the button on tabs that
   still close on middle-click.
3. npx vitest run src/components/pane-shell/tree/renderer/tab-close-affordance.test.tsx
   — the files and session-tile:abc cases fail. The test detects a button
   that stops tracking the verb.
4. Revert step 2. All four cases pass again.

In the app: hover the SESSIONS and BOTS tabs. Neither shows an ✕, and neither
closes on middle-click or ⌘-click, the same as before this PR. Hover a files
or session tile tab. Each shows an ✕ and closes on middle-click, the same as
before this PR.

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass — N/A for the Python suite (no Python file changes). The JS suite is the gate here: the full apps/desktop vitest run gives 6954 pass, 2 fail. The 2 failures are electron/remote-lifecycle.test.ts and electron/update-handoff-marker.test.ts. They fail the same way on a clean tree on this machine (nix sandbox spawn paths), and they do not touch the pane shell.
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: NixOS (Linux 7.1.8)

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — the
  PaneTab.onClose and PaneChrome.hideOnly doc comments now carry the
  rule. No user-facing doc mentions the flag.
• I've updated cli-config.yaml.example if I added/changed config keys — N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — renderer-only change with no platform-conditional code. ⌘-click and middle-click already route through lib/middle-click.ts for every host.
• I've updated tool descriptions/schemas if I changed tool behavior — N/A

Tests

tab-close-affordance.test.tsx renders the real TreeGroup and asserts one
contract over every tab kind that shares a strip:

expect(hasCloseButton(paneId)).toBe(middleClickCloses(paneId))

It reads closure from the layout tree (the pane is gone), not from a spy, so a
wired-up mock cannot pass it. Cases: hideOnly chrome, a plain side pane, the
uncloseable workspace, and a mirrored session tile.

The test carries no fixture for the deleted prop, because the compiler already
rejects it: passing showCloseButton to PaneTab now fails with TS2322: Property 'showCloseButton' does not exist. A test tombstone would only restate
that, under a name the codebase still uses for the unrelated DialogContent
prop. The remaining cases still catch the regression they exist for: hiding the
button on a closeable tab fails the files and session-tile cases.

Screenshots / Logs

$ npx vitest run src/components/pane-shell src/components/ui/pane-tab.test.tsx
 Test Files  29 passed (29)
      Tests  154 passed (154)

$ npm run typecheck
tsc -p . --noEmit && tsc -p tsconfig.electron.json --noEmit && tsc -p tsconfig.e2e.json --noEmit
(clean)

$ npm run lint
✖ 116 problems (0 errors, 116 warnings)

The single warning on the new file is no-restricted-globals for document.
Every sibling DOM probe in that directory carries the same warning.

[github-actions]

૮ >ﻌ< ა ci review

_{ran on 6e20855 — refactor(desktop): derive the tab hover close button from th}

⚠️ Warnings

OSV vulnerability scan · View job

7 known vulnerabilities found in pinned dependencies.

• CVE-2026-67213 in package-lock.json
• CVE-2026-67213 in website/package-lock.json
• CVE-2026-71554 in uv.lock
• CVE-2026-70608 in package-lock.json
• CVE-2026-56876 in package-lock.json
• CVE-2026-70606 in package-lock.json
• CVE-2026-71554 in uv.lock

How to fix:

Review the findings in the Security tab. Update the affected dependencies if a patched version is available.

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debug info

CI timings

CI timings · View report · View job

Wall time 4m20s vs 6m42s (-35.3%). 16 job(s) slower, 3 faster,

• JS & TS checks / apps/desktop / check:test:ui:shard-3of3: -54.0s
• JS & TS checks / apps/desktop / check:test:ui:shard-1of3: +41.0s
• JS & TS checks / apps/desktop / check:lint: +22.0s
• JS & TS checks / apps/desktop / check:test:ui:shard-2of3: +19.0s
• JS & TS checks / apps/desktop / check:test:desktop:all: +15.0s