Keyboard shortcuts (spacebar) no longer works in Photos on Mojave
Since MacOS Mojave, Photos have changed shortcuts. I used Space key very extensively, so made work again (link):


Go to preferences > keyboard > shortcuts > app shortcuts > Photos > and add a shortcut. 
For Menu Title enter "Open Viewer" and for the keyboard shortcut press the spacebar whilst holding the Fn key. 
Add another one for "Close Viewer"
Add another one for "Open/Close Viewer"
Now open Photos, go to Library/Photos, select single photo and click on menu "Image" - check if Open Viewer shows "Space" 
Press Spacebar and enjoy :)


Also few useful shortcuts:


cmd+Enter - edit mode
cmd+1 - in edit mode Adjust
cmd+3 - in edit mode Crop
. - make photo favorite


Also check cmd+1-4 and ctrl+1-6 in photos view mode :)

And desert: All Photos keyboard shortcuts for Mojave