Sveltekit isn't respecting the ssr = false setting?

I'm just starting with sveltekit so this is probably something I'm doing wrong, but I have a very basic page based on the SplitPane example in Svelte (https://svelte.dev/repl/5ab84358dd8b46ad9474884f2359ff9b?version=3.50.1) that isn't working because it's rendering on the server and not finding a window object:

   window is not defined   ReferenceError: window is not defined       at HSplitPane.svelte:60:8

The code lives under routes in "+page.svelte":

<script>    export const ssr = false;    import { HSplitPane } from 'svelte-split-pane';</script><div class="wrapper"><HSplitPane leftPaneSize="75%" rightPaneSize="25%" minLeftPaneSize="50px" minRightPaneSize="50px"><left slot="left"><h1>Welcome to SvelteKit</h1><p>Visit <a href="https://kit.svelte.dev">kit.svelte.dev</a> to read the documentation</p></left><right slot="right"></right></HSplitPane></div><style>    main {        text-align: center;        margin: 0 auto;    }    div.wrapper {        width: 95%;        height: 400px;        margin: auto;    }    left, right, top, down {        width: 100%;        height: 100%;        display: block;        text-align: center;    }    left, top {        background-color:coral    }    right, down {        background-color: cornflowerblue;    }</style>

I was under the impression that the first bit:

export const ssr = false

would take care of the error, but that doesn't seem to be doing it. Any help greatly appreciated.