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Extended SPACE Key Page Scroller
By default the SPACE key scrolls the page down by full height of browser view. With this script, pressing SHIFT+SPACE will scroll half of the view height. Page scroll by a quarter view height can be done using either LEFTSHIFT+RIGHTSHIFT+SPACE or SHIFT+CAPSLOCK+SPACE (configurable via variable).
Older Chrome version (e.g. v43) is not yet capable of differentiating between left and right SHIFT key. i.e. it only recognizes one type of SHIFT key.
Avoid using LEFTSHIFT+RIGHTSHIFT+SPACE for QuarterScrollKey for now, since all web browsers don't properly generate
keyup event when both SHIFT keys are held then released.