UIScaler

local collection_service = game:GetService("CollectionService")
local GUI_service = game:GetService("GuiService")
local workspace_service = game:GetService("Workspace")

local scale_component_to_base_resolution: {[UIScale]: Vector2} = {}
-- Required property `AbsoluteContentSize` is not in `UILayout` superclass.
type scrolling_frame_layout_component = UIGridStyleLayout | UIListLayout
local layout_component_to_rescaling_connection: {[scrolling_frame_layout_component]: RBXScriptConnection?} = {}

local camera = workspace_service:FindFirstChild("Camera")
local actual_viewport_size: Vector2? = nil

local has_initialized = false

local function rescale(scale_component: UIScale, base_resolution: Vector2)
    assert(actual_viewport_size ~= nil)
    scale_component.Scale = 1 / math.max(base_resolution.X / actual_viewport_size.X, base_resolution.Y / actual_viewport_size.Y)
end

local function rescale_all()
    local top_inset, bottom_inset = GUI_service:GetGuiInset()
    actual_viewport_size = camera.ViewportSize - top_inset - bottom_inset

    for scale_component, base_resolution in scale_component_to_base_resolution do
        rescale(scale_component, base_resolution)
    end
end

local function register_scale(component: UIScale)
    local base_resolution = component:GetAttribute("base_resolution")
    assert(typeof(base_resolution) == "Vector2")
    scale_component_to_base_resolution[component] = base_resolution
end

local function register_scrolling_frame_layout_component(layout_component: scrolling_frame_layout_component)
    local scale_component_referral = layout_component:FindFirstChild("scale_component_referral")
    assert(scale_component_referral:IsA("ObjectValue"))
    assert(scale_component_referral.Value ~= nil)
    assert(scale_component_referral.Value:IsA("UIScale"))
    local scale_component: UIScale = scale_component_referral.Value
    local scrolling_frame = layout_component.Parent
    assert(scrolling_frame ~= nil)
    assert(scrolling_frame:IsA("ScrollingFrame"))
    -- https://devforum.roblox.com/t/automaticcanvassize-working-with-uilistlayout-and-uiscale-causes-wrong-automatic-size/1334861.
    layout_component_to_rescaling_connection[layout_component] = layout_component:GetPropertyChangedSignal(
        "AbsoluteContentSize"
    ):Connect(function()
        scrolling_frame.CanvasSize = UDim2.fromOffset(
            layout_component.AbsoluteContentSize.X / scale_component.Scale,
            layout_component.AbsoluteContentSize.Y / scale_component.Scale
        )
    end)
    scrolling_frame.CanvasSize = UDim2.fromOffset(
        layout_component.AbsoluteContentSize.X / scale_component.Scale,
        layout_component.AbsoluteContentSize.Y / scale_component.Scale
    )
    print('set')
end

local function initialize()
    assert(has_initialized == false)
    has_initialized = true

    collection_service:GetInstanceAddedSignal("scale_component"):Connect(function(object: Instance)
        assert(object:IsA("UIScale"))

        register_scale(object)
        rescale(object, scale_component_to_base_resolution[object])
    end)
    collection_service:GetInstanceRemovedSignal("scale_component"):Connect(function(object: Instance)
        assert(object:IsA("UIScale"))
        scale_component_to_base_resolution[object] = nil
    end)

    for _, object in collection_service:GetTagged("scale_component") do
        assert(object:IsA("UIScale"))

        register_scale(object)
    end

    collection_service:GetInstanceAddedSignal("scrolling_frame_layout_component"):Connect(function(object: Instance)
        assert(object:IsA("UIGridStyleLayout") or object:IsA("UIListLayout"))
        register_scrolling_frame_layout_component(object)
    end)
    for _, object in collection_service:GetTagged("scrolling_frame_layout_component") do
        assert(object:IsA("UIGridStyleLayout") or object:IsA("UIListLayout"))
        register_scrolling_frame_layout_component(object)
    end
    collection_service:GetInstanceRemovedSignal("scrolling_frame_layout_component"):Connect(function(object: Instance)
        assert(object:IsA("UIGridStyleLayout") or object:IsA("UIListLayout"))
        local rescaling_connection = layout_component_to_rescaling_connection[object]
        assert(rescaling_connection ~= nil)
        rescaling_connection:Disconnect()
        layout_component_to_rescaling_connection[object] = nil
    end)

    camera:GetPropertyChangedSignal("ViewportSize"):Connect(rescale_all)
    rescale_all()
end

initialize()