pressing 'z' in thumbnail mode has strange results.

[0ion9]

Feh appears to switch to list mode, with a random image active.
But actually it's still in thumbnail mode -- pressing the left/right cursor keys visibly navigates through 'thumbnails' (which are now just areas on the screen, rather than actual previews), and those 'thumbnails' can still be clicked on or activated with Enter.

IMO the correct behaviour is to stay fully in thumbnail mode, moving to a random item.