Issue Type: Bug

Open a new window, close the Untitled tab, open Release notes and press Ctrl+W (or Cmd+W) on macOS.

Expected:
Release notes tab is closed.

Actual:
Release notes tab is closed as well as the current window.

You can also open a file, open Release notes and press Ctrl+W. Release notes and the last file will close.

VS Code version: Code 1.39.2 (6ab5985, 2019-10-15T15:33:40.634Z)
OS version: Linux x64 4.15.0-65-generic