This userscript is designed for mobile browsers, and scrolls page on double tap. Top half of the screen scrolls up, and bottom half scrolls down. Single taps are ignored for a specified timeout while script waits for a second tap, and then get dispatched programmatically.

Default value is to scroll page by 80% of the screen width on each double tap. It can be changed by modifying the amountToScroll variable (default value: 0.8)