The left side pane window, the one that can be "split vertically" must be undockable.

Otherwise, out of the 8 possilble pane types that the user can choose, on a regular full HD screen at most 3 are usable at the same time.
Making these panes undockable allows to have more of them floating e.g., in a second screen, and leaves more space for the editors in the main window.