Currently exiting the slide show using the mouse is not possible. I've just had a look at the mouse and keyboard navigation in the slide show.
Some inconsistencies and suggestions:
1 - When viewing images in fullscreen, the fullscreen mode can be exited using the middle mouse key. In slide show, the middle mouse key does nothing.
-> Middle mouse key should exit the slide show. Double click could also exit the slide show (not all users have a mouse with three buttons).
2 - Currently the transition to the next slide is triggered by timer or user. When timer is used, the mouse is disabled, but the keyboard isn't.
-> If transition is triggered by timer, there should be an extra option
[ ] Disable user interaction
Once this option is triggered, both mouse and keyboard should be disabled. Only Esc key (or even a user definable key combination) should exit the slide show.
3 - In image view mode, Page up & Page down navigates to the previous/Next image. And Pos1/End key to the first and last image. In slide show this does not work. (Cursor up/down/right/left can be used to navigate, only)
-> Page/Up down and Pos1/End should all be supported for navigating in slide show.
These are just some ideas to make usage easier, input is welcome.
Not to forget existing posts and suggestions: See
Keys in Slideshow?,
How to zoom in slideshow?.