I’m posting this because it deals with the multiple-pane problem discussed on the mailing list.
Here’s a work-in-progress of the LibreOffice menu mock-up.
Multiple panes would be managed under the “Layout” dropdown — you would select the open documents you’d like to open under several panes in a single window, click “Layout”, select the pane layout.
To separate a single window with multiple panes back into several windows, you’d select that window and choose Layout -> Separate panes into windows.
(Note: arrows pointing to the right indicate that the button expands as a pane on the right.)