You can try page up and down as replacement for your scroll function.
' Page Up '
EVENT TYPE=KEYPRESS SELECTOR=* KEY=33

' Page Down '
EVENT TYPE=KEYPRESS SELECTOR=* KEY=34

And in a row:
' Page Up '
EVENTS TYPE=KEYPRESS SELECTOR=* KEYS="[33,33,33,33,33,33,33,33,33,33,33]"

' Page Down '
EVENTS TYPE=KEYPRESS SELECTOR=* KEYS="[34,34,34,34,34,34,34,34,34,34,34]"

It's like 0.88 * window.innerHeight:
' Page Up with Javascript '
URL GOTO=javascript:window.scrollBy(0,-0.88*window.innerHeight);

' Page Down with Javascript '
URL GOTO=javascript:window.scrollBy(0,0.88*window.innerHeight);

And with arrow keys:
' Up (arrow)
EVENTS TYPE=KEYPRESS SELECTOR=* KEYS="[38,38,38,38,38,38,38]"

' Down (arrow)
EVENTS TYPE=KEYPRESS SELECTOR=* KEYS="[40,40,40,40,40,40,40]"