hammerspoon_marble_trackball_scroll.lua

-- Source:https://github.com/owenlo/Marble-Hammerspoon
-- Source:https://github.com/tekezo/Karabiner/issues/814#issuecomment-415388742

-- HANDLE SCROLLING WITH MOUSE BUTTON PRESSED
local backMouseButton = 3
local forwardMouseButton = 4
local deferred = false
local VSCODE = 'Code'
local VIM = 'MacVim'

function setOverrides(e)
    overrideOtherMouseDown:stop()
    overrideOtherMouseUp:stop()
    hs.eventtap.otherClick(e:location(), 0, pressedMouseButton)
    overrideOtherMouseDown:start()
    overrideOtherMouseUp:start()	
end

overrideOtherMouseDown = hs.eventtap.new({ hs.eventtap.event.types.otherMouseDown }, function(e)
    -- print("down")    
    local pressedMouseButton = e:getProperty(hs.eventtap.event.properties['mouseEventButtonNumber'])
    if backMouseButton == pressedMouseButton or forwardMouseButton == pressedMouseButton then
        deferred = true
        return true
    end
end)

overrideOtherMouseUp = hs.eventtap.new({ hs.eventtap.event.types.otherMouseUp }, function(e)
    -- print("up")
    local pressedMouseButton = e:getProperty(hs.eventtap.event.properties['mouseEventButtonNumber'])
    local appName = hs.application.frontmostApplication():name()

    if backMouseButton == pressedMouseButton then 
        if (deferred) then
            setOverrides(e)
            if appName == VSCODE then
                hs.eventtap.keyStroke({"ctrl"}, "-")
            elseif appName == VIM then

            else
                hs.eventtap.keyStroke({"cmd"}, "[")
            end
            return true
        end
        return false
    end

    if forwardMouseButton == pressedMouseButton then 
        if (deferred) then
            setOverrides(e)
            if appName == VSCODE then
                hs.eventtap.keyStroke({"ctrl", "shift"}, "-")
            elseif appName == VIM then
            else
                hs.eventtap.keyStroke({"cmd"}, "]")
            end
            return true
        end
        return false
    end
    return false
end)

local oldmousepos = {}
local scrollmult = -4	-- negative multiplier makes mouse work like traditional scrollwheel

dragOtherToScroll = hs.eventtap.new({ hs.eventtap.event.types.otherMouseDragged }, function(e)
    local pressedMouseButton = e:getProperty(hs.eventtap.event.properties['mouseEventButtonNumber'])
    -- print ("pressed mouse " .. pressedMouseButton)
    if backMouseButton == pressedMouseButton or forwardMouseButton == pressedMouseButton then 
        -- print("scroll");
        deferred = false
        oldmousepos = hs.mouse.absolutePosition()    
        local dx = e:getProperty(hs.eventtap.event.properties['mouseEventDeltaX'])
        local dy = e:getProperty(hs.eventtap.event.properties['mouseEventDeltaY'])
        local scroll = hs.eventtap.event.newScrollEvent({dx * scrollmult, dy * scrollmult},{},'pixel')
        -- put the mouse back
        hs.mouse.absolutePosition(oldmousepos)
        return true, {scroll}
    else 
        return false, {}
    end 
end)

overrideOtherMouseDown:start()
overrideOtherMouseUp:start()
dragOtherToScroll:start()