libinput plugin for touchpad / trackpad scrolling speed control. Vibe coded.

90-magic-trackpad-scroll-speed.lua

-- Reduce Magic Trackpad two-finger scroll speed by scaling multitouch motion.
-- The curve is { touchpad-coordinate-units-per-ms, multiplier }.
-- Low speeds stay precise; fast swipes are slowed less.
local SCROLL_SPEED_CURVE = {
    { 0.0, 0.25 },
    { 1.0, 0.25 },
    { 4.0, 0.40 },
    { 10.0, 0.65 },
    { 25.0, 0.90 },
}

local APPLE_VENDOR_ID = 0x004c
local MAGIC_TRACKPAD_BLUETOOTH_PRODUCT_ID = 0x0265

libinput:register({1})

local function round(value)
    if value >= 0 then
        return math.floor(value + 0.5)
    end

    return math.ceil(value - 0.5)
end

local function scroll_multiplier(speed)
    if speed <= SCROLL_SPEED_CURVE[1][1] then
        return SCROLL_SPEED_CURVE[1][2]
    end

    for i = 1, #SCROLL_SPEED_CURVE - 1 do
        local p1 = SCROLL_SPEED_CURVE[i]
        local p2 = SCROLL_SPEED_CURVE[i + 1]

        if speed >= p1[1] and speed <= p2[1] then
            local t = (speed - p1[1]) / (p2[1] - p1[1])

            return p1[2] + t * (p2[2] - p1[2])
        end
    end

    return SCROLL_SPEED_CURVE[#SCROLL_SPEED_CURVE][2]
end

local function slot_count(slots)
    local count = 0

    for _, slot in pairs(slots) do
        if slot.active then
            count = count + 1
        end
    end

    return count
end

local function reset_virtual_positions(slots)
    for _, slot in pairs(slots) do
        slot.x_virtual = slot.x
        slot.y_virtual = slot.y
        slot.x_initialized = false
        slot.y_initialized = false
    end
end

local function connect_scroll_scaler(device)
    local slots = {}
    local current_slot = 0
    local scaling_active = false
    local last_timestamp = 0

    device:connect("evdev-frame", function (_, frame, timestamp)
        local changed = false
        local dt_ms = 16.0

        if last_timestamp ~= 0 then
            dt_ms = (timestamp - last_timestamp) / 1000.0

            if dt_ms < 1.0 then
                dt_ms = 1.0
            elseif dt_ms > 200.0 then
                dt_ms = 200.0
            end
        end

        last_timestamp = timestamp

        for _, event in ipairs(frame) do
            if event.usage == evdev.ABS_MT_SLOT then
                current_slot = event.value
                slots[current_slot] = slots[current_slot] or {}
            elseif event.usage == evdev.ABS_MT_TRACKING_ID then
                local slot = slots[current_slot] or {}
                slots[current_slot] = slot

                if event.value == -1 then
                    slot.active = false
                    slot.x_initialized = false
                    slot.y_initialized = false
                else
                    slot.active = true
                    slot.x_initialized = false
                    slot.y_initialized = false
                end

                local fingers = slot_count(slots)

                local should_scale = fingers == 2

                if scaling_active ~= should_scale then
                    reset_virtual_positions(slots)
                    scaling_active = should_scale
                end
            elseif event.usage == evdev.ABS_MT_POSITION_X or event.usage == evdev.ABS_MT_POSITION_Y then
                local slot = slots[current_slot] or {}
                slots[current_slot] = slot

                local axis = event.usage == evdev.ABS_MT_POSITION_X and "x" or "y"
                local initialized = axis .. "_initialized"

                if slot.active and slot[initialized] and scaling_active then
                    local physical = slot[axis]
                    local virtual = slot[axis .. "_virtual"]
                    local delta = event.value - physical
                    local speed = math.abs(delta) / dt_ms
                    local scaled = virtual + delta * scroll_multiplier(speed)

                    slot[axis] = event.value
                    slot[axis .. "_virtual"] = scaled
                    event.value = round(scaled)
                    changed = true
                else
                    slot[axis] = event.value
                    slot[axis .. "_virtual"] = event.value
                    slot[initialized] = true
                end
            end
        end

        if changed then
            return frame
        end

        return nil
    end)

    device:connect("device-removed", function (removed_device)
        removed_device:disconnect("evdev-frame")
        removed_device:disconnect("device-removed")
    end)
end

libinput:connect("new-evdev-device", function (device)
    local info = device:info()

    if info.vid ~= APPLE_VENDOR_ID or info.pid ~= MAGIC_TRACKPAD_BLUETOOTH_PRODUCT_ID then
        return
    end

    local usages = device:usages()

    if usages[evdev.ABS_MT_SLOT]
        and usages[evdev.ABS_MT_TRACKING_ID]
        and usages[evdev.ABS_MT_POSITION_X]
        and usages[evdev.ABS_MT_POSITION_Y] then
        connect_scroll_scaler(device)
        libinput:log_info("Magic Trackpad scroll speed curve enabled")
    end
end)

Disclaimer: I'm not familiar with libinput, its plugin system, or Lua. I've just updated to Gnome 50, and since they added the support for the libinput plugins, I've asked ChatGPT to fix the scrolling speed for me. It gave me this plugin.

Some instructions for Gnome 50+:

1. Put the file into ~/.config/libinput/plugins/ (create if it doesn't exist)
2. Check your touchpad/trackpad ID, and replace those in the script if needed.
3. Set the curve how you like it.
4. Log out and log back in.

To check the trackpad ID, use libinput list-devices. For me it looks like this:

Device:                  Apple Inc. Magic Trackpad
Kernel:                  /dev/input/event23
Id:                      bluetooth:004c:0265

And bluetooth:004c:0265 becomes this:

local APPLE_VENDOR_ID = 0x004c
local MAGIC_TRACKPAD_BLUETOOTH_PRODUCT_ID = 0x0265

---

Libinput 1.30.0 or higher is required, as this is the version the plugin system was introduced. I guess if you want to apply the plugin globally, you put the file into /etc/libinput/plugins/.

If I understand correctly from the MR#4739, the ~/.config/libinput/plugins/ directory is enabled by Gnome itself and can take no effect in other Wayland compositors, so check the docs for your environment.