@humanspeak/svelte-virtual-chat

A high-performance virtual chat viewport for Svelte 5. Purpose-built for LLM conversations, support chat, and any message-based UI. Renders only visible messages to the DOM while maintaining smooth follow-bottom behavior, streaming stability, and history prepend with anchor preservation.

Why This Exists

Chat UIs are not generic lists. They have specific behaviors that general-purpose virtual list components handle poorly:

• Messages anchor to the bottom, not the top
• New messages should auto-scroll when you're at the bottom
• Scrolling away should not snap you back when new messages arrive
• LLM token streaming causes messages to grow in height mid-render
• Loading older history should preserve your scroll position

@humanspeak/svelte-virtual-chat is opinionated about these behaviors so you don't have to fight a generic abstraction.

Features

• Bottom gravity — messages sit at the bottom of the viewport, like every chat app
• Follow-bottom — viewport stays pinned to the newest message while at bottom
• Scroll-away stability — new messages don't yank you back when you've scrolled up
• Virtualized rendering — only visible messages exist in the DOM (handles 10,000+ messages)
• Streaming-native — height changes from LLM token streaming are batched per frame
• Header & footer — optional snippets for persistent content above/below messages (typing indicators, banners)
• History prepend — load older messages at the top without viewport jumping
• Message-aware — uses message IDs for identity, not array indices
• Full TypeScript — strict types, generics, and exported type definitions
• Svelte 5 runes — built with $state, $derived, $effect, and snippets
• Debug info — real-time stats via onDebugInfo callback (total, DOM count, measured, range, following state)
• E2E tested — 66+ Playwright tests across 7 test suites
• Zero dependencies — only esm-env for SSR detection

Requirements

• Svelte 5
• Node.js 18+

Installation

# Using pnpm (recommended)
pnpm add @humanspeak/svelte-virtual-chat

# Using npm
npm install @humanspeak/svelte-virtual-chat

# Using yarn
yarn add @humanspeak/svelte-virtual-chat

Basic Usage

<script lang="ts">
    import SvelteVirtualChat from '@humanspeak/svelte-virtual-chat'

    type Message = { id: string; role: string; content: string }

    let messages: Message[] = $state([
        { id: '1', role: 'assistant', content: 'Hello! How can I help?' },
        { id: '2', role: 'user', content: 'Tell me about Svelte.' }
    ])
</script>

<div class="h-[600px]">
    <SvelteVirtualChat
        {messages}
        getMessageId={(msg) => msg.id}
        estimatedMessageHeight={72}
        containerClass="h-full"
        viewportClass="h-full"
    >
        {#snippet renderMessage(message, index)}
            <div class="p-4 border-b">
                <strong>{message.role}</strong>
                <p>{message.content}</p>
            </div>
        {/snippet}
    </SvelteVirtualChat>
</div>

Props

| Prop | Type | Default | Description | | ------------------------- | -------------------------------------------- | -------- | ------------------------------------------------------- | | messages | TMessage[] | Required | Array of messages in chronological order (oldest first) | | getMessageId | (msg: TMessage) => string | Required | Extract a unique, stable ID from a message | | renderMessage | Snippet<[TMessage, number]> | Required | Snippet that renders a single message | | header | Snippet | - | Persistent content above all messages (always in DOM) | | footer | Snippet | - | Persistent content below all messages (always in DOM) | | estimatedMessageHeight | number | 72 | Height estimate in pixels for unmeasured messages | | followBottomThresholdPx | number | 48 | Distance from bottom to consider "at bottom" | | overscan | number | 6 | Extra messages rendered above/below the viewport | | onNeedHistory | () => void \| Promise<void> | - | Called when user scrolls near top (load older messages) | | onFollowBottomChange | (isFollowing: boolean) => void | - | Called when follow-bottom state changes | | onDebugInfo | (info: SvelteVirtualChatDebugInfo) => void | - | Called with live stats on every scroll/render update | | containerClass | string | '' | CSS class for the outermost container | | viewportClass | string | '' | CSS class for the scrollable viewport | | debug | boolean | false | Enable console debug logging | | testId | string | - | Base test ID for data-testid attributes |

Imperative API

Bind the component to access these methods:

<script lang="ts">
    let chat: ReturnType<typeof SvelteVirtualChat>
</script>

<SvelteVirtualChat bind:this={chat} ... />

<button onclick={() => chat.scrollToBottom({ smooth: true })}> Scroll to bottom </button>

| Method | Signature | Description | | ----------------- | ------------------------------------------------------ | --------------------------------------------------- | | scrollToBottom | (options?: { smooth?: boolean }) => void | Scroll the viewport to the bottom | | scrollToMessage | (id: string, options?: { smooth?: boolean }) => void | Scroll to a specific message by its ID | | isAtBottom | () => boolean | Check if the viewport is currently following bottom | | getDebugInfo | () => SvelteVirtualChatDebugInfo | Get a snapshot of current debug stats |

LLM Streaming

The component handles streaming natively. As a message grows token by token, ResizeObserver detects the height change and the viewport stays pinned to bottom without jitter.

Pair with @humanspeak/svelte-markdown for rich markdown rendering with streaming support:

<script lang="ts">
    import SvelteVirtualChat from '@humanspeak/svelte-virtual-chat'
    import SvelteMarkdown from '@humanspeak/svelte-markdown'

    type Message = {
        id: string
        role: 'user' | 'assistant'
        content: string
        isStreaming?: boolean
    }

    let messages: Message[] = $state([...])
</script>

<SvelteVirtualChat
    {messages}
    getMessageId={(msg) => msg.id}
    containerClass="h-[600px]"
    viewportClass="h-full"
>
    {#snippet renderMessage(message, index)}
        <div class="p-4 border-b">
            {#if message.role === 'assistant'}
                <SvelteMarkdown source={message.content} streaming={message.isStreaming ?? false} />
            {:else}
                <p>{message.content}</p>
            {/if}
        </div>
    {/snippet}
</SvelteVirtualChat>

History Loading

Load older messages when the user scrolls near the top. The component preserves the user's scroll position during prepend operations.

<script lang="ts">
    import SvelteVirtualChat from '@humanspeak/svelte-virtual-chat'

    let messages = $state([...recentMessages])
    let isLoading = false

    async function loadHistory() {
        if (isLoading) return
        isLoading = true
        const older = await fetchOlderMessages()
        messages = [...older, ...messages]
        isLoading = false
    }
</script>

<SvelteVirtualChat
    {messages}
    getMessageId={(msg) => msg.id}
    onNeedHistory={loadHistory}
    containerClass="h-[600px]"
    viewportClass="h-full"
>
    {#snippet renderMessage(message, index)}
        <div class="p-4">{message.content}</div>
    {/snippet}
</SvelteVirtualChat>

Header & Footer

Add persistent content above and below messages without injecting fake entries into the messages array. Both snippets are always in the DOM (never virtualized) and scroll with the content.

Footer height changes automatically trigger follow-bottom snapping — perfect for typing indicators:

<SvelteVirtualChat
    {messages}
    getMessageId={(msg) => msg.id}
    containerClass="h-[600px]"
    viewportClass="h-full"
>
    {#snippet header()}
        <div class="p-3 text-center text-sm text-gray-500">Beginning of conversation</div>
    {/snippet}
    {#snippet renderMessage(message, index)}
        <div class="p-4 border-b">
            <p>{message.content}</p>
        </div>
    {/snippet}
    {#snippet footer()}
        {#if isTyping}
            <div class="p-3 text-sm text-gray-500">Assistant is typing...</div>
        {/if}
    {/snippet}
</SvelteVirtualChat>

Debug Info

The onDebugInfo callback provides real-time visibility into the component's internal state:

<script lang="ts">
    import SvelteVirtualChat from '@humanspeak/svelte-virtual-chat'
    import type { SvelteVirtualChatDebugInfo } from '@humanspeak/svelte-virtual-chat'

    let stats: SvelteVirtualChatDebugInfo | null = $state(null)
</script>

<SvelteVirtualChat
    {messages}
    getMessageId={(msg) => msg.id}
    onDebugInfo={(info) => (stats = info)}
    ...
/>

{#if stats}
    <div>
        Total: {stats.totalMessages} | In DOM: {stats.renderedCount} | Following: {stats.isFollowingBottom}
    </div>
{/if}

| Field | Type | Description | | ------------------- | --------- | ---------------------------------------- | | totalMessages | number | Total messages in the array | | renderedCount | number | Messages currently in the DOM | | measuredCount | number | Messages with measured heights | | startIndex | number | First rendered index | | endIndex | number | Last rendered index | | totalHeight | number | Calculated total content height (px) | | scrollTop | number | Current scroll position (px) | | viewportHeight | number | Viewport height (px) | | isFollowingBottom | boolean | Whether the viewport is pinned to bottom | | averageHeight | number | Average measured message height (px) |

TypeScript

Full type exports for building typed wrappers and extensions:

import type {
    SvelteVirtualChatProps,
    SvelteVirtualChatDebugInfo,
    ScrollToBottomOptions,
    ScrollToMessageOptions,
    VisibleRange,
    ScrollAnchor
} from '@humanspeak/svelte-virtual-chat'

// Utility exports
import {
    ChatHeightCache,
    captureScrollAnchor,
    restoreScrollAnchor
} from '@humanspeak/svelte-virtual-chat'

How Virtualization Works

The component uses standard top-to-bottom geometry (no inverted lists):

1. Height caching — Each message's height is measured via ResizeObserver and cached by ID
2. Visible range — On every scroll, the component calculates which messages fall within scrollTop to scrollTop + viewportHeight, plus an overscan buffer
3. Absolute positioning — Only visible messages are rendered, positioned via transform: translateY() inside a content div sized to the total calculated height
4. Follow-bottom — When at bottom, new messages and height changes trigger an automatic snap to scrollHeight
5. Bottom gravity — When messages don't fill the viewport, flex-direction: column; justify-content: flex-end pushes them to the bottom

With 10,000 messages, the DOM contains ~15-25 elements instead of 10,000.

Performance

| Metric | Value | | ------------------------------ | ---------------------------------------- | | DOM nodes with 1,000 messages | ~15-25 (viewport + overscan) | | DOM nodes with 10,000 messages | ~15-25 (same) | | Follow-bottom snap | Single requestAnimationFrame per batch | | Height measurement | ResizeObserver (no polling) | | Streaming height updates | Batched per animation frame |

Companion Libraries

| Package | Description | | ------------------------------------------------------------------------------------------------ | ------------------------------------------------------------- | | @humanspeak/svelte-markdown | Markdown renderer with LLM streaming mode (~1.6ms per update) | | @humanspeak/svelte-virtual-list | General-purpose virtual list for non-chat use cases |

License

MIT © Humanspeak, Inc.

Credits

Made with ❤️ by Humanspeak