Tap on the cat pit to put the image in viewer mode (for pinch zooming, etc) -- this hides the navbar to display just the image
Hit the Android back button to go back to the details screen
The Material navbar will still be hidden
The navbar will only come back if you again tap on the cat pic in details, returning you to the image view screen, then tap on the image to show the navbar in view mode.
Repro steps:
The navbar will only come back if you again tap on the cat pic in details, returning you to the image view screen, then tap on the image to show the navbar in view mode.
This seems to be a bug in F7, but I will confirm.