scrollbar.lua

function frame_scrollbar(content_height)
    local frame = get_current_frame()
    local frame_origin = get_frame_origin()
    local frame_size = get_frame_size()
    local frame_height = frame_size.y
    local scrollbar_height_relative = frame_height / content_height

    if scrollbar_height_relative >= 1 then
        return
    end

    local scrollbar_offset_relative = frame.scroll_position / content_height
    local scrollbar_height = scrollbar_height_relative * frame_height
    local scrollbar_size = vector(10, scrollbar_height)
    local scrollbar_position = frame_origin + vector(frame_size.x - scrollbar_size.x, scrollbar_offset_relative * frame_height)

    local id = "frame_scrollbar_" .. tostring(#frame_stack)
    local was_active = is_active(id)
    local scrollbar = button_behavior(id, make_rectangle(scrollbar_position, scrollbar_size))
    local became_active = scrollbar.active and not was_active

    if became_active then
        frame.scroll_drag_offset = ui_state.mouse_position.y - scrollbar_position.y
    end

    local function clamp_scroll_space(scroll_position_content_space)
        return math.max(0, math.min(scroll_position_content_space, content_height - frame_height))
    end

    if scrollbar.active then
        local scroll_position_screen_space = ui_state.mouse_position.y - frame_origin.y - frame.scroll_drag_offset
        local scroll_position_content_space = scroll_position_screen_space / frame_height * content_height

        frame.scroll_position = clamp_scroll_space(scroll_position_content_space)

        love.graphics.setColor(0.7, 0.7, 0.7)
    elseif scrollbar.hovered then
        love.graphics.setColor(0.9, 0.9, 0.9)
    else
        love.graphics.setColor(1, 1, 1)
    end

    love.graphics.rectangle("fill", scrollbar_position.x, scrollbar_position.y, scrollbar_size.x, scrollbar_size.y)

    local mouse_scroll = ui_state.wheel_input.y * 50

    if mouse_scroll ~= 0 and rectangle_contains(frame.rectangle, ui_state.mouse_position) then
        frame.scroll_position = clamp_scroll_space(frame.scroll_position - mouse_scroll)
    end
end

i like it although i have not yet tested