window.html

<style>
    html {
        height: 100%;
    }

    #window_id {
        position: absolute;
        -webkit-user-select: none;
        border: 1px solid #000;
        font-size: 12px;
        left: 0%;
        top: 0%;
        min-width:200pt;
        min-height:200pt;
    }

    #window_id main {
        overflow: auto;
        max-height: 50vh;
    }

    #window_id .border {
        border: 1px dotted #000;
        line-height: 24px;
        cursor: move;
    }
</style>

<div class="window" id="window_id">
    <div class="border">border</div>
    <main>main</main>
</div>