subwindows.lua

local imgui = load_imgui({version="1.0.0", name="test"})

function OnWorldPreUpdate()
    if imgui.Begin("Main Window Title") then

        local available_width, available_height = imgui.GetContentRegionAvail()

        if imgui.BeginChild("TopLeft", 0.2 * available_width, 200, true) then
            imgui.Text("TopLeft")
            imgui.EndChild()
        end

        imgui.SameLine()


        local tr_flags = bit.bor(
            imgui.WindowFlags.MenuBar
        )

        if imgui.BeginChild("TopRight", 0.8 * available_width, 200, true, tr_flags) then

            if imgui.BeginMenuBar() then
                if imgui.BeginMenu("Files") then

                    if imgui.MenuItem("New") then
                        print("Clicked on new file!")
                    end

                    imgui.EndMenu()
                end
                imgui.EndMenuBar()
            end

            for i=1,20 do
                imgui.Text("TopRight" .. i)
            end
            imgui.EndChild()
        end

        if imgui.BeginChild("DownLeft", 0.2 * available_width, 0, true) then
            imgui.Text("DownLeft")
            imgui.EndChild()
        end

        imgui.SameLine()

        if imgui.BeginChild("DownRight", 0.8 * available_width, 0, true) then
            imgui.Text("DownRight")
            imgui.EndChild()
        end

        imgui.End()
    end
end