Listener leak in chatEditingEditorOverlay causes Extension Host crash when Copilot Chat is open alongside Flutter/Dart project

[Tarek-higazi]

• Copilot Chat Extension Version: 0.40.1
• VS Code Version: 1.112.0
• OS Version: macOS
• Feature: Edit mode
• Selected model: Claude Sonnet 4.6
• Logs:
  "ERR [1c9] potential listener LEAK detected, having 323 listeners already. MOST frequent listener (89):: Error
  at a.create (event.ts:1061:15)
  at Yoi._event [as onDidChange] (event.ts:1246:34)
  at new tr (toolbar.ts:382:1)
  at a._createInstance (instantiationService.ts:162:18)
  at a.createInstance (instantiationService.ts:128:18)
  at new AG (chatEditingCodeEditorIntegration.ts:760:1)
  at a._createInstance (instantiationService.ts:162:18)
  at a.createInstance (instantiationService.ts:128:18)
  at chatEditingCodeEditorIntegration.ts:450:1
  at a.invokeFunction (instantiationService.ts:109:11)
  at Ur.invokeWithinContext (codeEditorWidget.ts:442:1)
  at chatEditingCodeEditorIntegration.ts:448:1
  at MOo (viewZones.ts:418:10)
  at viewZones.ts:223:4
  at a.changeWhitespace (linesLayout.ts:204:4)
  at ySt.changeWhitespace (viewLayout.ts:378:40)
  at MSt.changeWhitespace (viewModelImpl.ts:1254:38)
  at Qvt.changeViewZones (viewZones.ts:202:27)
  at $ye.change (view.ts:720:19)
  at Ur.changeViewZones (codeEditorWidget.ts:1658:1)
  at PG._updateDiffRendering (chatEditingCodeEditorIntegration.ts:352:1)
  at KW._runFn (chatEditingCodeEditorIntegration.ts:200:1)
  at KW._run (autorunImpl.ts:110:11)
  at new KW (autorunImpl.ts:58:8)
  at Se (autorun.ts:18:9)
  at new PG (chatEditingCodeEditorIntegration.ts:184:1)
  at a._createInstance (instantiationService.ts:162:18)
  at a.createInstance (instantiationService.ts:128:18)
  at Xie._createEditorIntegration (chatEditingModifiedDocumentEntry.ts:307:1)
  at Xie.getEditorIntegration (chatEditingModifiedFileEntry.ts:313:1)
  at Qv._computeFn (chatEditingEditorOverlay.ts:365:1)
  at Qv._recompute (derivedImpl.ts:201:24)
  at Qv.get (derivedImpl.ts:159:11)
  at KW.readObservable (autorunImpl.ts:225:28)
  at Qv.read (baseObservable.ts:54:18)
  at KW._runFn (chatEditingEditorOverlay.ts:169:1)
  at KW._run (autorunImpl.ts:110:11)
  at new KW (autorunImpl.ts:58:8)
  at Se (autorun.ts:18:9)
  at ftt.show (chatEditingEditorOverlay.ts:166:1)
  at KW._runFn (chatEditingEditorOverlay.ts:373:1)
  at KW._run (autorunImpl.ts:110:11)
  at KW.endUpdate (autorunImpl.ts:167:12)
  at Qv.endUpdate (derivedImpl.ts:281:7)
  at Qv.endUpdate (derivedImpl.ts:281:7)
  at VN.finish (transaction.ts:102:13)
  at Qi (transaction.ts:21:6)
  at AYt.handleEvent (observableSignalFromEvent.ts:41:3)
  at tSe.value (event.ts:194:85)
  at E._deliver (event.ts:1338:13)
  at E._deliverQueue (event.ts:1349:9)
  at E.fire (event.ts:1373:9)
  at editorGroupView.ts:1269:1
  at async Kw.openAnything (anythingQuickAccess.ts:1130:1)"

Steps to Reproduce:

1. Open a Flutter/Dart project in VS Code with Copilot Chat enabled
2. Open Copilot Chat and send any message
3. Switch between editor tabs a few times
4. Open Help > Toggle Developer Tools > Console and observe
   "potential listener LEAK detected" errors growing to 300-400+ listeners
5. Eventually the extension host crashes with "Debug adapter process
   has terminated unexpectedly (Extension host shut down)",
   killing the Flutter debug session

Root cause visible in stack trace:
_autoAttachInstructions → findAgentSkills → computeSkillDiscoveryInfo
→ chatEditingEditorOverlay.show() → _createEditorIntegration
→ _updateDiffRendering → new toolbar created but never disposed,
leaking ~89-131 listeners per tab switch.

Regression noticed in 1.111.0, still present in 1.112.0.