I don't believe there is currently a method for this.....could an option for a standard title bar be added to Legend native MacOS? Currently, the only location that can be quickly grabbed with the mouse is the zone above the Boards pane, between the colored minimize/maximize buttions and the "Resize Sidebar" pulldown button. If that column is very narrow that leaves no place for a mouse drag of the main form.
Adding an option to Settings (that would have to be specific to each OS Platform) to allow a standard Title Bar would be more intuitive and in line with other applications.