Hide view container tabs in secondary sidebar when only one container present

[Copilot]

When the secondary sidebar contains only a single view container, the tab bar serves no purpose and wastes vertical space.

Changes

Modified auxiliaryBarPart.ts to hide the composite bar (view container tabs) when ≤1 container is visible:

• shouldShowCompositeBar(): Returns false when getVisiblePaneCompositeIds().length ≤ 1
• Event listeners: Added handlers for onDidChangeViewContainers, onDidChangeContainerLocation, and onDidPaneCompositeOpen/Close to dynamically update visibility as containers are added/removed/moved

The tabs now appear/disappear automatically as view containers are added to or removed from the secondary sidebar. Primary sidebar and panel behavior unchanged.

Testing

1. Move single view container (e.g., Chat) to secondary sidebar → tabs hidden
2. Add second container → tabs appear
3. Remove until one remains → tabs hide
4. Verify behavior with activity bar at different positions (top/bottom/hidden/default)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Hide view container tabs in secondary sidebar when only one view container is present</issue_title>
<issue_description>When I'm using the secondary sidebar with only a single view (like Chat), the view container tabs still take up valuable vertical space even though there's nothing to switch between.

Proposal:
When there's only one view container in the secondary sidebar, hide the view container tabs entirely since they serve no purpose.

Benefits:

• Maximizes available space for the actual content
• Reduces visual clutter when tabs aren't needed
• Better use of screen real estate, especially for chat interactions

This should help make better use of the available space when the secondary sidebar contains a single view.</issue_description>

Comments on the Issue (you are @copilot in this section)

@sbatten I use activity bar on bottom with icons btw

• Fixes Hide view container tabs in secondary sidebar when only one view container is present #282794

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