Previously, left-clicking with the mouse would advance the presentation to the next page no matter the cursor position. With this patch, clicks on the right half of the screen still lead to the next page being shown, but clicks on the left half lead to the previous page now.
As touch events are set to create mouse events, touching on the left half of a touch screen will lead to the previous page, too. This should make people happy that use Okular for sheet music.
The actual action happens on mouse-release. For touch screens this means less interference with swipe gestures (which also allow to go to the previous/next page).