A chat scroll container that anchors turns, opens saved transcripts, follows streamed responses, loads history without jumping, and jumps to any message.
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What Makes a Great Streaming Chat Experience
Building a chat interface used to be simple. You create an inverted list with an input. Type a message, it appends at the bottom. When a reply comes in, the list grows and scrolls. Done.
Streaming breaks that model. Messages arrive in chunks while you may still be reading, scrolling, or looking somewhere else entirely.
Now the challenge is preserving the reader's place while the conversation keeps changing. Get that wrong and the experience feels jumpy: people are pulled to the bottom, lose context, and have to find their way back.
In practice, this comes down to scroll: when to follow, when to hold, and when to let the reader decide. A great streaming chat should:
- Move only when the reader asked to move. If someone is reading, don’t pull them somewhere else. Auto-scroll should never be the default.
- Follow only while they’re following. If they’re at the live edge, keep the stream in view. If they scroll away, leave them there.
- Every interaction is a signal. Scrolling is not the only one. Selecting text, using the keyboard, opening a link, or searching should all stop the interface from moving.
- Start a new turn near the top of the viewport. This gives the new turn somewhere it can be read from the beginning.
- Then stream in the answer. The answer should grow into the screen, not immediately push everything away.
- Keep part of the previous conversation in context. The prompt and reply should stay visually connected, and enough of the previous turn should remain visible so the reader knows where they are.
- Let new content arrive offscreen. The conversation can keep streaming without changing what the reader is looking at.
- Show what’s happening out of view. Make it clear when a response is still streaming or when new messages have arrived.
- Make it easy to return to the latest reply. A “Jump to latest” action should bring the reader back and resume following.
- Let people jump anywhere in the conversation. Long threads need message links, search, unread markers, and direct navigation.
- Reopen where the reader left off. A saved conversation should open at the last meaningful turn. Often this is the last user message. Not the absolute bottom.
- Keep the reader’s place when layout changes. Images load. Markdown expands. Code blocks render. Older messages appear above. None of that should make the reader lose their place.
- Handle interruptions without stealing position. Stopping, retrying, regenerating, branching, or errors should not unexpectedly move the conversation.
- Stay responsive in long threads. Streaming text, markdown, code, images, and long history should still feel responsive.
- Be accessible without the noise. Keep the transcript navigable, preserve keyboard focus, and announce important events at a comfortable pace.
Never move the reader against their intent.
MessageScroller
MessageScroller is a chat transcript scroller built for these behaviors. MessageScroller.Provider owns the scroll state and transcript-row behavior:
opening position, streamed output, new-turn anchoring, prepended history,
visibility, and scroll controls. MessageScroller.Root is the styled frame that
renders inside it.
MessageScroller is scoped to the scroll viewport. It does not own messages, AI state, transport, persistence, branching, or model state. Your product code stays focused on composing messages, markers, tools, attachments, and prompt inputs.
It gives you the scroll behavior that chat needs, without taking over the rest of the chat UI. And it stays fast, even in long conversations with rich markdown.
Installation
Install @shadcn-svelte/primitives:
Copy and paste the following code into your project.
Usage
<script lang="ts">
import * as Message from "$lib/components/ui/message/index.js";
import * as MessageScroller from "$lib/components/ui/message-scroller/index.js";
</script><MessageScroller.Provider>
<MessageScroller.Root>
<MessageScroller.Viewport>
<MessageScroller.Content>
{#each messages as message (message.id)}
<MessageScroller.Item
messageId={message.id}
scrollAnchor={message.role === "user"}
>
<Message.Root />
</MessageScroller.Item>
{/each}
</MessageScroller.Content>
</MessageScroller.Viewport>
<MessageScroller.Button />
</MessageScroller.Root>
</MessageScroller.Provider>MessageScroller fills its parent, so place it inside a height-constrained
container.
<div class="flex h-screen flex-col">
<MessageScroller.Provider>
<MessageScroller.Root class="flex-1">
<!-- transcript -->
</MessageScroller.Root>
</MessageScroller.Provider>
</div>Composition
<MessageScroller.Provider>
<MessageScroller.Root>
<MessageScroller.Viewport>
<MessageScroller.Content>
<MessageScroller.Item>
<!-- a message, marker, or row -->
</MessageScroller.Item>
<MessageScroller.Item />
<MessageScroller.Item />
</MessageScroller.Content>
</MessageScroller.Viewport>
<MessageScroller.Button />
</MessageScroller.Root>
</MessageScroller.Provider>MessageScroller.Provider— the headless root. Owns scroll state and the behavior props for opening position, auto-scroll, anchoring, scroll commands, and visibility tracking.MessageScroller.Root— the styled frame. Lays out the viewport, content, and controls inside the provider.MessageScroller.Viewport— the scrollable element. Receives native scroll events and preserves the visible row when older messages are prepended.MessageScroller.Content— the transcript container. Holds the rows and provides the live-region defaults for new messages.MessageScroller.Item— the transcript row boundary. Wrap every direct child of the content so the scroller can measure, anchor, preserve position, track visibility, and jump to it. An item can be a message, marker, typing indicator, separator, join/leave event, or "load earlier" row.MessageScroller.Button— the scroll control. Scrolls to the start or end of the transcript and is inert until there is content in its direction.
Core Concepts
Anchoring Turns
A turn is the part of the conversation that starts a new exchange. In a simple AI chat, that is usually the user's message and the assistant reply that follows.
An anchor is the row the viewport should treat as the start of that turn. Mark
that row with scrollAnchor. When a new anchor is appended, the viewport moves
it near the top and keeps a peek of the previous item above it, so the new turn
does not feel detached from its context.
<!-- This tells the scroller to anchor the user's message for the next turn. -->
<MessageScroller.Item
messageId={message.id}
scrollAnchor={message.role === "user"}
/>Scroll anchors are not tied to message role. You can turn any row into an anchor:
a user message, a system marker, a handoff event, or anything else that starts a
meaningful turn. MessageScroller only needs to know which row should anchor the
viewport.
In the following example, the user's message is anchored. When you send a new message, the viewport anchors it near the top and appends the assistant reply below it. Toggle the anchor to the assistant's message to see the difference.
Choose which role settles near the top edge.
Group Chat
In a group chat, the turn boundary is more specific than "the user message". It is often the message that asks the model to respond, or a marker like "Marcus joined the chat". Typing indicators and history controls usually should not anchor.
Because anchoring is role-independent, you can anchor a marker just as easily as a message.
<MessageScroller.Item messageId="marcus-joined" scrollAnchor>
<Marker.Root variant="separator">
<Marker.Content>Marcus joined the chat</Marker.Content>
</Marker.Root>
</MessageScroller.Item>A group chat with several participants and an assistant. The Marker is marked as a turn.
Keeping Context Visible
When a new turn starts, it should still feel like part of the same continuous
thread. scrollPreviousItemPeek keeps a slice of the previous item visible
above the anchor, so the reader keeps their context instead of feeling like the
conversation restarted on a blank page.
<!-- Keep 64px of the previous turn visible above the newly anchored row. -->
<MessageScroller.Provider scrollPreviousItemPeek={64}>
<MessageScroller.Root>
<!-- anchored turns -->
</MessageScroller.Root>
</MessageScroller.Provider>Adjust the peek amount in the example below to see how it affects the conversation.
New turns keep part of the previous reply in view.
Following the Live Edge
When the reader is at the live edge, either because they stayed there or
returned there, autoScroll keeps streamed replies in view as they grow.
Scrolling away from the live edge releases the view, whether by wheel, touch,
keyboard scroll keys, or dragging the scrollbar. An explicit message jump
releases it too. New chunks can then arrive without moving the reader.
autoScroll composes with turn anchoring. When a new turn anchors near the
top, the view stays put while the reply streams into the room below it. Once
the reply fills the viewport, the reader is back at the live edge and
follow-output takes over from the anchor.
<MessageScroller.Provider autoScroll>
<MessageScroller.Root>
<!-- streamed turns -->
</MessageScroller.Root>
</MessageScroller.Provider>Auto-scroll follows the live edge of the conversation.
Calling scrollToEnd, or pressing MessageScroller.Button, re-engages
follow-output when autoScroll is enabled, so a reader who scrolled away can
return to the live edge and keep following. The root and viewport expose data-autoscrolling while that programmatic scroll to the latest message runs,
so you can conditionally apply styles during the transition.
Opening Saved Threads
It can seem reasonable to reopen a saved thread at the absolute end of the
transcript, but that often drops the reader into the conversation without enough
context. A better default is "last-anchor": show the last meaningful turn,
like the user's latest message, with the reply below it.
That gives the reader an immediate place in the thread. They can see what they asked, where the answer starts, and continue from there without reconstructing the conversation from the bottom edge.
<MessageScroller.Provider defaultScrollPosition="last-anchor">
<MessageScroller.Root>
<!-- transcript -->
</MessageScroller.Root>
</MessageScroller.Provider>Choose where a saved transcript opens.
"last-anchor" is keyed on scrollAnchor, not message role. If no anchor
exists, or the last anchored turn already fits in the viewport, it falls back to "end".
Use "start" when you want to resume at the beginning of a conversation, or "end" when the absolute latest message is the right place to land.
Avoiding a Flash on Reload
A scroll container always opens at the top. HTML has no way to set scrollTop,
so a server-rendered transcript shows the oldest messages first. After
JavaScript runs, defaultScrollPosition moves the view, and you see a jump.
When defaultScrollPosition is "end" or "last-anchor", the viewport has data-pending-scroll until that position is applied. The styled viewport stays
hidden while the attribute is present, so you see the frame instead of the jump. "start" does not need this.
If you want "end" visible on first paint, add an inline script right after the
viewport. Give the viewport an id, scroll it to the bottom, and remove data-pending-scroll.
<script>
const scrollToEndScript = `(function () {
var viewport = document.getElementById("messages")
if (!viewport) {
return
}
viewport.scrollTop = viewport.scrollHeight
viewport.removeAttribute("data-pending-scroll")
})()`;
</script>
<MessageScroller.Root>
<MessageScroller.Viewport id="messages">
<MessageScroller.Content>
<!-- transcript -->
</MessageScroller.Content>
</MessageScroller.Viewport>
{@html `<script>${scrollToEndScript}</script>`}
<MessageScroller.Button />
</MessageScroller.Root>Put the script in your page, not in the scroller. It only works for "end", and
only when the messages are already in the HTML. If you use a Content Security Policy, pass a nonce.
Do not use this script with "last-anchor". Skip it when messages load on the
client.
Loading Earlier Messages
Loading earlier messages should not move the conversation the reader is already
looking at. When older rows are prepended above the current transcript, MessageScroller.Viewport preserves the visible row so the reader stays in the
same place while history loads above them.
This is enabled by default through preserveScrollOnPrepend.
Prepended messages keep your place.
Use stable messageId values for message rows. That gives the scroller a
specific row to preserve instead of guessing from whichever pixel happens to sit
at the viewport edge.
Animating New Messages
MessageScroller.Item can be animated directly. Keep messageId and scrollAnchor on it, and use transform and opacity for the entrance.
A common chat pattern is to animate the user's message when it is sent, then let the assistant reply stream into a regular row below it. Start the user row below its final position so it feels like it rises from the live edge of the viewport.
<MessageScroller.Item
class="animate-in fade-in slide-in-from-bottom-2 duration-300"
messageId={message.id}
scrollAnchor
/>Choose how user messages are animated when they are added to the conversation.
Avoid animating height, margin, or padding for row entrances; those changes can fight the scroller's positioning work. If the reader prefers reduced motion, skip the entrance animation and keep the scroll behavior the same.
Jumping to Messages
Search results, permalinks, outline items, and toolbar buttons often need to
drive the transcript from outside the message list. Use useMessageScroller for
those controls. Because the hooks read from MessageScroller.Provider, they work
in any component inside the provider, including controls rendered outside the MessageScroller.Root frame.
<script lang="ts">
import { useMessageScroller } from "$lib/components/ui/message-scroller/index.js";
const { scrollToMessage, scrollToEnd, scrollToStart } = useMessageScroller();
</script>Drive the transcript from outside.
scrollToMessage targets the messageId on MessageScroller.Item, so rows that
need to be addressable should have stable ids. scrollToMessage returns false when the target is not mounted and cannot be queued.
scrollToMessage can queue a target before items exist, which covers
client-resolved permalinks while the transcript mounts. After rows have mounted,
a missing id returns false instead of starting a guessed retry loop. A true result means the scroll ran or was queued, not that the row is already in view.
Tracking the Reader's Position
Use useMessageScrollerVisibility to track the reader's position in the
conversation. A common example is a table-of-contents or a jump menu that
highlights the current anchored turn.
<script lang="ts">
import { useMessageScrollerVisibility } from "$lib/components/ui/message-scroller/index.js";
const visibility = useMessageScrollerVisibility();
</script>Track the current anchored turn.
currentAnchorId answers "where am I" by reporting the current anchored turn,
and it stays set after that anchor scrolls above the viewport. visibleMessageIds answers "what is on screen", in document order.
Visibility is pay-for-what-you-use. Tracking only runs while something
subscribes to useMessageScrollerVisibility, and rows need a messageId to
participate.
Reading Scroll State
Use useMessageScrollerScrollable when you need scroll state in JavaScript, such
as a status indicator or a custom "jump to latest" control. It reports which
edges the viewport can still scroll toward; "at the start/end" is the negation
(!start / !end), and "scrollable at all" is start || end. For styling the
scroller itself, prefer the data-scrollable attribute.
<script lang="ts">
import { useMessageScrollerScrollable } from "$lib/components/ui/message-scroller/index.js";
const scrollable = useMessageScrollerScrollable();
</script>Where the reader can go scroll to based on current scroll position.
Performance
MessageScroller is benchmarked against large transcripts with markdown and
composed message rows.
The scroll hot path stays outside of Svelte state: no transcript-row rerenders for scroll, no forced layout on every scroll, and as little off-screen paint work as the browser can avoid.
Scroll position, anchoring, and follow-output are tracked imperatively and mirrored onto the root and viewport through data-* attributes, so scrolling and streaming do not rerender transcript rows.
The styled MessageScroller.Item also ships with content-visibility: auto and contain-intrinsic-size. Rows stay in the DOM for selection, copy,
find-in-page, SSR, and assistive tech, but the browser can skip rendering work
for rows far outside the viewport.
Visibility tracking is pay-for-what-you-use. A jump menu or active
turn indicator costs nothing until something subscribes to useMessageScrollerVisibility.
This is comfortable for the expected range of a chat transcript: hundreds to low thousands of turns, including messages with markdown and composed components.
Virtualization
Virtualization is intentionally left outside the primitive. MessageScroller renders real DOM rows and stays fast well into the thousands of turns (see Performance), so most transcripts never need it.
When a transcript is large enough to need virtualization, use MessageScroller.Viewport as the scroll element and let the virtualizer own the
rows.
<script lang="ts">
import { useVirtualizer } from "@shadcn-svelte/primitives/svelte-virtual";
import * as Message from "$lib/components/ui/message/index.js";
import * as MessageScroller from "$lib/components/ui/message-scroller/index.js";
let {
messages,
}: {
messages: Array<{ id: string; content: import("svelte").Snippet }>;
} = $props();
let viewport = $state<HTMLDivElement | null>(null);
const virtualizer = useVirtualizer({
get count() {
return messages.length;
},
getScrollElement: () => viewport,
estimateSize: () => 86,
getItemKey: (index) => messages[index]?.id ?? index,
overscan: 8,
});
</script>
<MessageScroller.Provider>
<MessageScroller.Root>
<MessageScroller.Viewport bind:ref={viewport}>
<MessageScroller.Content class="block min-h-full">
<div
class="relative w-full"
style="height: {virtualizer.getTotalSize()}px"
>
{#each virtualizer.getVirtualItems() as virtualItem (virtualItem.key)}
{@const message = messages[virtualItem.index]}
{#if message}
<div
data-index={virtualItem.index}
class="absolute start-0 top-0 w-full"
style="transform: translateY({virtualItem.start}px)"
{@attach (node) => virtualizer.measureElement(node)}
>
<Message.Root>{@render message.content()}</Message.Root>
</div>
{/if}
{/each}
</div>
</MessageScroller.Content>
</MessageScroller.Viewport>
<MessageScroller.Button />
</MessageScroller.Root>
</MessageScroller.Provider>Accessibility
MessageScroller keeps the scroll container keyboard reachable and the
transcript announceable without forcing a specific message UI.
MessageScroller.Viewport is a labelled, keyboard-focusable scroll region by
default. It uses role="region", aria-label="Messages", and tabindex={0},
so keyboard users can focus the transcript and scroll it directly.
MessageScroller.Content marks the transcript as a live region with role="log" and aria-relevant="additions". New rows can be announced, but
streamed text mutations do not have to be announced token by token.
<MessageScroller.Content aria-busy={status === "streaming"}>
<!-- messages -->
</MessageScroller.Content>Pass aria-busy while a turn streams if announcements should wait for the
completed message row.
MessageScroller.Button renders a real button. When there is nothing to scroll
toward, it sets inert, uses tabindex={-1}, and exposes data-active="false" so inactive scroll controls do not create extra focus stops.
Unstyled
The behavior in MessageScroller comes from the @shadcn-svelte/primitives package. Import the
headless parts from @shadcn-svelte/primitives/message-scroller when you want your own markup
and styles.
API Reference
The props, data attributes, and hooks for every part live on the unstyled MessageScroller primitives in @shadcn-svelte/primitives/message-scroller. They are identical
for the styled component and the unstyled parts.