in the normal development view I can switch between "before" and "after" using Shift-v (using German as language, should be \ in English), I also can see a text "Before" next to the picture when the before-picture is shown.
In fullscreen preview (F) it seems I also can use Shift-v: I get the text "Before" but the picture does not change.
Is this a bug?
Using Win7-x64, problem occured both with LR5.4 and LR5.5.
Hmm, I get somewhat different behavior in my LR 5.6 (English).
- On Mac OS 10.9, when I enter Full Screen Preview (F) from Develop mode, typing "\" does nothing.
- But on Windows 7, typing "\" exits full-screen preview back into Develop mode, with the "Before" label in the upper-right corner.
Which platform are you on?
You can file a bug report in the official feedback forum here: New topic for Photoshop Family.
Sorry, forgot that and edited some minutes later: Win7 x64, LR5.5
Try with LR 5.6 before filing a bug report. (Adobe appears to have been making changes to Full Screen Previous in the last minor releases.)