When in single photo viewing mode, a filmstrip is placed at the bottom of the central view area. This filmstrip helps to navigate between photos of a current group (Event, Album, search results, etc.), staying in the context without having to switch back to the thumbnails view.
The visibility and height of the filmstrip can be adjusted.
To show/hide the filmstrip, hit the space bar on your keyboard.
To change the height of the filmstrip, use the slider at the bottom panel, on the right side.