The Preview panel is layered on top, hiding other windows

[ronmurp]

When the markdown editor is open, showing the editing and preview panels, the preview panel remains in the foreground when another window is opened over the markdown editor.

To Reproduce

• Open the markdown editor, pinned, in a 'New Horizontal Document Group', showing both edit and preview panels.
• Open the Output window, unpinned, so that it overlays the editing windows.
  See image.

Expected behavior
The output window should overlay the editor panels.

Screenshots

[madskristensen]

Duplicate of #49