The Preview pane can be re-positioned to a different part of the screen or undocked entirely in order to display on a second monitor.
To re-position, click and drag the top bar, outlined with a red box in the figure below, of the Preview pane to another location.
For example, if you would like to relocate the Preview pane to where the Navigation pane now resides, click and drag to the shaded icon on the left (
) and release.
The Preview pane now appears on the left, and the Navigation and Detail panes have been rearranged accordingly.
The location of the preview pane, or if it is docked/undocked status can also be managed by choosing Preview Pane from the View menu.