Have you ever wondered how to lock a photo in Lightroom while continue analysing or working on developing your photos in the main screen Library or Develop modules. Well I discovered today that there is a function button just for that. By pressing F11 on your computer, another window will open and you can select what to see in it whether the Grid, Loup, Compare, or Survey views in this separate window.
In Loupe view you can Lock the second window so that you see a single image in it regardless of what is visible on the main screen or choose Normal to view the currently selected image or Live to see the image under the mouse pointer.
To close this window, just press F11 again or click the upper right 'X' button.