Hello bitdream
It is actually not a bug, you just need to apply some css to avoid this issue. In layout.svelte file try this css and wrap the fiemanager content into the div's this way:
 
<div class="layout">
	<div class="content">
			{@render children()}
	 </div>
</div>
<style>
    .layout {
        height: 100%;
        width: 100%;
        overflow: hidden;
    }
    .content {
        height: 100%;
        width: calc(100% - 80px);
        transition:
            transform 0.3s,
            width 0.3s;
        transform: translateX(80px);
        overflow-y: auto;
        position: relative;
    }
</style>
And in the main app.html (or index.html) add this style:
 
 <style>
      html,
      body {
        margin: 0;
        padding: 0;
        height: 100%;
      }
    </style>
Please check the example: https://stackblitz.com/edit/sveltejs-kit-template-default-bkb6bt6z?description=The%20default%20SvelteKit%20template,%20generated%20with%20create-svelte&file=src%2Fapp.html&title=SvelteKit%20Default%20Template