I’m filing this as a BUG.
I deal a lot with images, and use Quicklook to step through them in the Finder. I normally have two monitors, but sometimes have only one.
In the Finder, using Quicklook fullscreen is much more convenient with two monitors, but can be done with one.
Here’s how I do it. In the Finder, go to a folder with photos in it (I use column view, or list: icon view won’t work) and select a photo, then tap the space bar to start Quicklook. I then click the expand icon on the image to go fullscreen. Now, however, you can’t navigate through the folder with the images in it. The trick is to type Cmd-~ (or Cmd-` more accurately) which switches focus between open windows in an app. In this case it switches focus from the fullscreen image to the folder hidden behind it. You can now use the arrow keys to navigate up and down through the images to view them.
The BUG: This does not work in TotalFinder. The Cmd-~ does not switch focus from the picture to the folder behind it, and the arrow keys do nothing. I have to Relaunch into the Finder to use this technique.
Since I use this several times a day, it’s a real pain when I’m on a single-monitor setup.
In dual-monitor setup, you can arrange the folder on one monitor and the fullscreen Quicklook on the other, and just click to gain focus, so it’s not a problem there.
I hope this bug can be fixed.