A simple AutoHotKey script to implement the half-page-scrolling in SumatraPDF. Maybe as a rescue : )

sumatrapdf_ext.ahk

#NoEnv  ; Recommended for performance and compatibility with future AutoHotkey releases.
; #Warn  ; Enable warnings to assist with detecting common errors.
SendMode Input  ; Recommended for new scripts due to its superior speed and reliability.
SetWorkingDir %A_ScriptDir%  ; Ensures a consistent starting directory.

; --------Sumatra PDF hotkeys------------
; Add a half-page-scroll-up-like functionality

; Configure here
scrollAmount = 8 

; Function:
;   Check whether it is focusing on the PDF content, not other search input text boxes or windows.
;   From Window Spy, I found that the "Focus Control" field is surprisingly "" 
;   when we are focusing the content.
CheckFocusOnContent(){
	ControlGetFocus, fvar, ahk_class SUMATRA_PDF_FRAME
	if (StrLen(fvar) = 0) {
		return 1
	}else{
		return 0
	}
}

; These key maps only activate when we are in SumatraPDF and actually focus on the PDF content, 
; aka, not in the bookmark area or the search field
; I override the original "m" and "n" shortcuts here because I never use them.
#If WinActive("ahk_exe SumatraPDF.exe") and CheckFocusOnContent()
	m::Send  {WheelUp   %scrollAmount%} ; fast scroll up the "scrollAmount"
	n::Send  {WheelDown %scrollAmount%} ; fast scroll down the "scrollAmount"

This is no longer needed after installing the newer version of SumartraPDF.
We can make use of the new configuraiton like this:

Shortcuts [
	[
		Cmd = CmdScrollUpHalfPage
		Key = m
	]
	[
		Cmd = CmdScrollDownHalfPage
		Key = n
	]
]