AutoHotKey v2 script to activate mod keys and enable fast scrolling when mouse buttons are being held for more than 300ms.

mouse_mods.ahk

#Persistent
SetTimer, CheckHoldCtrl, Off ; Timer for Ctrl (forward button)
SetTimer, CheckHoldShift, Off ; Timer for Shift (back button)
SetTimer, CheckHoldAlt, Off ; Timer for Alt (middle button)
HoldTime := 0.3 ; Hold time threshold in milliseconds
ScrollSpeed := 10 ; Multiplier for faster scrolling


; Back button (XButton1) -> Shift key
XButton1::
    KeyWait, XButton1, T%HoldTime% ; Wait for the button to be released within the threshold
    if (ErrorLevel) {
        ; If the button is held for more than the threshold
        Send, {Shift Down} ; Press Shift
        SetTimer, CheckHoldShift, On ; Start a timer to monitor for release
    } else {
        ; Otherwise, it was a quick press
        Send, {XButton1} ; Send the back button event
    }
return

CheckHoldShift:
    if !GetKeyState("XButton1", "P") {
        ; If the back button is released
        if GetKeyState("Shift", "P") {
            Send, {Shift Up} ; Release Shift
        }
        SetTimer, CheckHoldShift, Off ; Stop monitoring
    }
return


; Forward button (XButton2) -> Ctrl key
XButton2::
    KeyWait, XButton2, T%HoldTime% ; Wait for the button to be released within the threshold
    if (ErrorLevel) {
        ; If the button is held for more than the threshold
        Send, {Ctrl Down} ; Press Ctrl
        SetTimer, CheckHoldCtrl, On ; Start a timer to monitor for release
    } else {
        ; Otherwise, it was a quick press
        Send, {XButton2} ; Send the forward button event
    }
return

CheckHoldCtrl:
    if !GetKeyState("XButton2", "P") {
        ; If the forward button is released
        if GetKeyState("Ctrl", "P") {
            Send, {Ctrl Up} ; Release Ctrl
        }
        SetTimer, CheckHoldCtrl, Off ; Stop monitoring
    }
return


; Middle mouse button (MButton) -> Alt key
MButton::
    KeyWait, MButton, T%HoldTime% ; Wait for the button to be released within the threshold
    if (ErrorLevel) {
        ; If the button is held for more than the threshold
        Send, {Alt Down} ; Press Alt
        SetTimer, CheckHoldAlt, On ; Start a timer to monitor for release
    } else {
        ; Otherwise, it was a quick press
        Send, {MButton} ; Send the middle mouse button event
    }
return

CheckHoldAlt:
    if !GetKeyState("MButton", "P") {
        ; If the middle mouse button is released
        if GetKeyState("Alt", "P") {
            Send, {Alt Up} ; Release Alt
        }
        SetTimer, CheckHoldAlt, Off ; Stop monitoring
    }
return


; Mouse wheel handling for faster scrolling when both buttons are held
*WheelUp::
    if (GetKeyState("XButton1", "P") && GetKeyState("XButton2", "P")) {
        if GetKeyState("Ctrl", "P") {
            Send, {Ctrl Up} ; Release Ctrl
        }
        if GetKeyState("Shift", "P") {
            Send, {Shift Up} ; Release Shift
        }
        Send, {WheelUp %ScrollSpeed%}
    } else {
        Send, {WheelUp}
    }
return

*WheelDown::
    if (GetKeyState("XButton1", "P") && GetKeyState("XButton2", "P")) {
        if GetKeyState("Ctrl", "P") {
            Send, {Ctrl Up} ; Release Ctrl
        }
        if GetKeyState("Shift", "P") {
            Send, {Shift Up} ; Release Shift
        }
        Send, {WheelDown %ScrollSpeed%}
    } else {
        Send, {WheelDown}
    }
return