Secondary Side Bar with Copilot Chat disappears when opening Markdown files

Does this issue occur when all extensions are disabled?: Yes

- VS Code Version: 1.136.0
- OS Version: macOS 26.6 (25G72)

## Steps to Reproduce

1. Open VS Code with the Copilot Chat view displayed in the **Secondary Side Bar**.
2. Open any Markdown (`.md`) file.
3. Observe that the **Secondary Side Bar containing Copilot Chat disappears automatically**.
4. Switch back to a non-Markdown file, such as a `.py` or `.ts` file.
5. Observe that the **Secondary Side Bar and Copilot Chat reappear automatically**.

## Expected Behavior

The **Secondary Side Bar should remain visible** when opening Markdown files, just as it does for other file types. Opening or switching to a Markdown editor should not automatically hide the Secondary Side Bar.

## Actual Behavior

When a Markdown file is opened, the **Secondary Side Bar disappears automatically**, including the Copilot Chat view that was previously visible.

When switching back to another file type, the **Secondary Side Bar reappears automatically**.

## Additional Information

The behavior appears to be specifically associated with opening Markdown files. The Secondary Side Bar state changes based on the active editor/file type without the user explicitly toggling the sidebar.

Screenshots demonstrating the behavior are attached.

with markdown file: