Hide primary sidebar when showing sessions sidebar on narrow windows

[Copilot]

When invoking Show Agent Sessions Sidebar on a narrow editor window, the action appeared as a no-op because there wasn't enough width to display the sidebar side-by-side.

Changes:

• When switching to side-by-side orientation and the main container width is insufficient, automatically hide the primary sidebar to make room
• Only hides the sidebar if doing so would actually provide enough space for the requested width

if (mainContainerWidth < newWidth && layoutService.isVisible(Parts.SIDEBAR_PART)) {
    const sidebarSize = layoutService.getSize(Parts.SIDEBAR_PART);
    if (mainContainerWidth + sidebarSize.width >= newWidth) {
        layoutService.setPartHidden(true, Parts.SIDEBAR_PART);
    }
}

Original prompt

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This section details on the original issue you should resolve

<issue_title>Show Agent Sessions Sidebar doesn't appear on narrower editor instance</issue_title>
<issue_description>

Ref https://github.com/microsoft/vscode-internalbacklog/issues/6527

• I was running two different instances of the same app in separate VS Code windows, side-by-side so that I could compare different agentic results on the same prompts
• ❓ Selecting Show Agent Sessions Sidebar didn't actually display the sidebar, which felt like an unexpected no-op (since I didn't think my editor instance was that narrow)

Delegation.1-8.4.mp4

I'm not sure if we can do much here, but I figured I'd still report it for discussion.

</issue_description>

<agent_instructions>When the available width is not sufficient for the sessions sidebar to show, hide the primary sidebar to make more room. This should be in src/vs/workbench/contrib/chat/browser/agentSessions/agentSessionsActions.ts</agent_instructions>

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

@bpasero A possible change to do here is to just maximize the 2nd sidebar in case the available space does not suffice.

• Fixes Show Agent Sessions Sidebar doesn't appear on narrower editor instance #286634

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[Copilot]

Pull request overview

This PR fixes an issue where invoking "Show Agent Sessions Sidebar" appeared as a no-op on narrow editor windows because there wasn't sufficient width to display the sidebar in side-by-side orientation.

Key Changes:

• Adds logic to detect when the auxiliary bar width constraint prevents proper side-by-side display after attempting to resize
• Automatically maximizes the auxiliary bar when the actual width after resize is insufficient for the minimum side-by-side width requirement