fix - UI freeze caused by deadlock in EditorsManager

[xinyi-gong]

fix #175

Problem

The UI freezes on startup when a user closes an editor tab while Copilot is still initializing.

Deadlock chain:

1. Worker thread calls getOrCreateNesRenderManager() → ConcurrentHashMap.computeIfAbsent() holds an internal bucket lock → inside the lambda, new RenderManager() → registerListeners() → Display.syncExec() blocks waiting for UI thread
2. UI thread handles editor close → disposeNesRenderManager() → ConcurrentHashMap.remove() blocks waiting for the same bucket lock

Both threads wait on each other indefinitely → permanent UI freeze.

Fix

Replace computeIfAbsent() with get() + putIfAbsent() so that the RenderManager construction (which calls syncExec) happens outside of any map lock. If two threads race to create a manager for the same editor, the loser disposes its instance and uses the winner's.

[Copilot]

Pull request overview

This PR fixes a startup UI freeze caused by a deadlock between a worker thread creating a RenderManager (which performs Display.syncExec()) and the UI thread removing from the same ConcurrentHashMap bucket in EditorsManager.

Changes:

• Replaced ConcurrentHashMap.computeIfAbsent() with get() + putIfAbsent() so RenderManager construction happens outside CHM’s internal bucket locks.
• Added a contention path that disposes the “losing” RenderManager instance and returns the already-registered instance.

Comments suppressed due to low confidence (1)

com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/completion/EditorsManager.java:176

• In the putIfAbsent contention path you dispose a freshly-created RenderManager. RenderManager currently subscribes to the Eclipse IEventBroker (NES accept/reject topics) but does not unsubscribe in dispose(), so a “losing” instance can keep receiving events for the same StyledText as the “winning” instance, causing duplicate accept/reject handling and memory leaks. Please ensure RenderManager.dispose() fully unregisters all listeners/subscriptions so disposing a losing instance is safe.

      mgr = new RenderManager(this.languageServer, this.nesProvider, editor);
      RenderManager existing = nesRenderManagers.putIfAbsent(editor, mgr);
      if (existing != null) {
        mgr.dispose();
        mgr = existing;