or3-scroll

A headless, chat-optimized virtual scroller for Vue 3. Designed for bottom-anchored layouts, streaming content, and dynamic item heights.

Features

• Smart Auto-Scroll: Locks to the bottom with a tight, configurable threshold (default 10px), allowing users to easily "break free" to read history without fighting the scroller.
• Stable Browsing: Commits layout changes at scroll end while preserving the visible keyed row and its within-row offset.
• Bottom Anchoring: Keeps the scroll position pinned to the bottom as new content arrives (chat style).
• Dynamic Heights: Handles items with variable and changing heights without jitter.
• Prepend Support: Seamlessly handles loading history (prepending items) while maintaining scroll position.
• Hidden Measurement: Measures items in a hidden pool before rendering to ensure accurate scroll offsets.
• Optimized Tail Rendering: Smart tail region handling with maxWindow constraint prevents excessive DOM nodes while keeping recent messages always rendered.
• Viewport Resize Handling: Gracefully handles container height changes (e.g., mobile keyboards) with ResizeObserver integration.
• Jump-to-Message: Built-in useScrollJump composable for ID-based navigation with partial history loading support.
• Media Prefetch Range: Warms media ahead of the viewport without mounting additional rows.
• SSR-Safe Import: Browser observers are constructed lazily after mounting.

Scroll Physics & Auto-Scroll Behavior

or3-scroll implements a sophisticated auto-scroll logic designed specifically for high-frequency chat applications:

1. Explicit Scroll Intent:

   • Following-bottom, user-browsing, programmatic-jump, and layout-compensation work are tracked separately.
   • Wheel, pointer, touch, keyboard, scrollbar, and unexplained upward movement suspend bottom following.
   • Reaching the bottom or calling scrollToBottom() resumes following when maintainBottom is enabled.

2. Committed Scrollbar & Keyed Anchor:

   • While browsing, model height changes are batched until native scrollend or the inactivity fallback fires.
   • The visible keyed row and within-row offset are preserved when the committed height changes.
   • While following, the new track height and bottom position are committed together.

3. Frame-Coalesced Work:

   • Native scroll events, range calculation, prefetch calculation, and the public scroll event are coalesced to the latest position once per animation frame.

Installation

npm install or3-scroll
# or
yarn add or3-scroll
# or
pnpm add or3-scroll
# or
bun add or3-scroll

Basic Usage

Here is a minimal example of a chat interface using <Or3Scroll>.

<script setup lang="ts">
import { ref } from 'vue';
import { Or3Scroll } from 'or3-scroll';
import 'or3-scroll/style.css'; // Required for structural layout

const messages = ref([
    { id: 1, text: 'Hello!' },
    { id: 2, text: 'How are you?' },
    // ... more messages
]);

const onReachTop = () => {
    console.log('Load more history...');
    // prepend logic here
};
</script>

<template>
    <div class="chat-container">
        <Or3Scroll
            :items="messages"
            item-key="id"
            :estimate-height="32"
            @reachTop="onReachTop"
        >
            <template #default="{ item }">
                <div class="message">
                    {{ item.text }}
                </div>
            </template>
        </Or3Scroll>
    </div>
</template>

<style>
.chat-container {
    height: 400px; /* Must have a fixed height */
}
.message {
    padding: 8px;
    border-bottom: 1px solid #eee;
}
</style>

Using Methods

To use exposed methods like scrollToBottom or scrollToItemKey, attach a ref to the component.

<script setup lang="ts">
import { ref } from 'vue';
import { Or3Scroll } from 'or3-scroll';

// 1. Create a ref typed with the component instance
const scrollRef = ref<InstanceType<typeof Or3Scroll> | null>(null);

const jumpToBottom = () => {
    // 2. Call methods on the ref
    scrollRef.value?.scrollToBottom();
};

const jumpToMessage = (id: number) => {
    scrollRef.value?.scrollToItemKey(id, { smooth: true, align: 'center' });
};
</script>

<template>
    <div class="controls">
        <button @click="jumpToBottom">Go to Bottom</button>
        <button @click="jumpToMessage(50)">Jump to #50</button>
    </div>

    <div class="chat-container">
        <Or3Scroll ref="scrollRef" :items="messages" item-key="id">
            <!-- ... -->
        </Or3Scroll>
    </div>
</template>

Jump to Message with History Loading

For "jump to message" functionality with partial history loading, use the useScrollJump composable:

<script setup lang="ts">
import { ref } from 'vue';
import { Or3Scroll, useScrollJump } from 'or3-scroll';

const messages = ref([...]);
const scrollerRef = ref<InstanceType<typeof Or3Scroll> | null>(null);

// Setup jump-to-message with history loading
const { jumpTo, jumpState } = useScrollJump({
  scrollerRef,
  items: messages,
  getItemId: (msg) => msg.id,
  loadHistoryUntil: async (targetId, direction) => {
    // Load messages until targetId is found
    while (!messages.value.find(m => m.id === targetId)) {
      const olderMessages = await fetchOlderMessages();
      if (olderMessages.length === 0) break;
      messages.value = [...olderMessages, ...messages.value];
    }
  }
});

// Later: jump to a message that might not be loaded yet
jumpTo('message-123', { align: 'center' });
</script>

<template>
    <div class="chat-container">
        <!-- Show a loading indicator while searching history -->
        <div v-if="jumpState.state === 'waitingForHistory'" class="jump-loader">
            Locating message...
        </div>

        <Or3Scroll ref="scrollerRef" :items="messages" item-key="id">
            <!-- ... -->
        </Or3Scroll>
    </div>
</template>

API Reference

Props

| Prop | Type | Default | Description | | --------------------- | ----------------------------------------- | ------- | ---------------------------------------------------------------------------------------------------------------- | | items | T[] | — | The array of data items to render. | | itemKey | keyof T \| ((item: T) => string \| number) | — | Required stable, unique key accessor. | | estimateHeight | number | 50 | Estimated row height in pixels. | | overscan | number | 200 | Extra pixels of mounted rows above and below the viewport. | | prefetchOverscan | number | 0 | Extra pixels used only for prefetchRange; it never mounts rows or changes track height. | | maintainBottom | boolean | true | Follows new tail content unless the user is browsing. | | loadingHistory | boolean | false | Enables measured prepend handling. | | tailCount | number | 0 | Number of tail rows kept mounted when the overscanned range reaches the tail. | | paddingBottom | number | 0 | Extra scroll-track padding below the rows. | | paddingTop | number | 0 | Extra scroll-track padding above the rows. | | bottomThreshold | number | 3 | Maximum distance used to report the physical bottom. | | autoscrollThreshold | number | 10 | Upward distance that breaks bottom-following intent. | | mutationMode | 'append-prepend' \| 'arbitrary' | 'append-prepend' | Selects streaming fast paths or full keyed reconciliation. | | contentKey | string \| number | — | Content epoch; changing it cancels stale work, resets measurements, and establishes the initial position. |

Slots

| Slot | Props | Description | | ----------------- | ---------------------------- | -------------------------------------------------------------------------------------------------- | | default | { item: T, index: number } | The content for each item. | | prepend-loading | - | Content to show at the top of the list when loadingHistory is true. Useful for loading spinners. |

Events

| Event | Payload | Description | | --------------- | ------------------------------------------------------- | ------------------------------------------------------------------ | | reachTop | — | Emitted once when crossing into the top boundary. | | reachBottom | — | Emitted once when crossing into the bottom boundary. | | scroll | { scrollTop, scrollHeight, clientHeight, isAtBottom } | Emitted at most once per animation frame. | | prefetchRange | { startIndex, endIndex } | Key-index range to warm without changing the rendered row window. |

Methods (Exposed via ref)

| Method | Arguments | Description | | --------------------- | ----------------------------------------------------------------------------------------------- | --------------------------------------------------------------- | | scrollToBottom | () | Scrolls to the bottom of the list. | | scrollToIndex | (index: number, options?: { align?: 'start' \| 'center' \| 'end', smooth?: boolean }) | Scrolls to a specific item index. | | scrollToItemKey | (key: string \| number, options?: { align?: 'start' \| 'center' \| 'end', smooth?: boolean }) | Scrolls to a specific item by its key. | | refreshMeasurements | () | Forces a re-measurement of all items. | | reset | () | Clears measurements and re-establishes the initial position. | | isAtBottom | boolean | Property indicating if the scroller is currently at the bottom. |

Recipes

Infinite History Preload

To implement infinite scrolling upwards (loading history):

1. Listen to the @reachTop event.
2. Fetch older messages.
3. Prepend them to your items array.
4. or3-scroll will automatically adjust the scroll position so the user doesn't lose their place.

const isLoadingHistory = ref(false);

const onReachTop = async () => {
    if (isLoadingHistory.value) return;
    isLoadingHistory.value = true;

    try {
        const olderMessages = await fetchHistory();
        messages.value = [...olderMessages, ...messages.value];
    } finally {
        isLoadingHistory.value = false;
    }
};

And in your template:

<Or3Scroll
    :items="messages"
    :loading-history="isLoadingHistory"
    @reachTop="onReachTop"
>
  <template #prepend-loading>
    <div class="spinner">Loading history...</div>
  </template>
  <!-- ... -->
</Or3Scroll>

AI Streaming Tail

For AI chat interfaces where the last message grows in real-time:

1. Ensure maintainBottom is true.
2. When the AI response updates, update the last item in your items array (immutably or deeply reactive).
3. The scroller will keep the bottom in view as the content expands.

Performance Tips

• Tail Count: Set tailCount to the number of recent messages you want always rendered (e.g., 10-20). This prevents flickering during rapid updates while keeping total DOM nodes low.
• Viewport Resize: The component automatically handles container height changes via ResizeObserver. On mobile, this means smooth behavior when the keyboard opens/closes.

Caveats / Gotchas

• Fixed Height Container: The parent container of <Or3Scroll> must have a fixed height (e.g., height: 100vh or height: 500px) and overflow: hidden (the component handles the scrolling internally).
• Images: If items contain images, their height might change after loading. It's best to define image dimensions explicitly or use the refreshMeasurements method after images load if you see layout shifts.
• ResizeObserver: The component uses ResizeObserver for viewport height tracking. This is supported in all modern browsers but not in test environments like JSDOM by default.

Security Considerations

⚠️ Important: This component renders user-provided content through Vue slots. Always sanitize and validate user-generated content before passing it to the component to prevent XSS (Cross-Site Scripting) attacks. The component itself does not perform any sanitization.

<!-- ❌ BAD: Rendering unsanitized user input -->
<Or3Scroll :items="messages">
  <template #default="{ item }">
    <div v-html="item.userContent"></div> <!-- Dangerous! -->
  </template>
</Or3Scroll>

<!-- ✅ GOOD: Sanitize user input or use text interpolation -->
<Or3Scroll :items="messages">
  <template #default="{ item }">
    <div>{{ item.userContent }}</div> <!-- Safe text interpolation -->
  </template>
</Or3Scroll>

Troubleshooting

Warnings

• [or3-scroll] Container has 0 height: The parent element of <Or3Scroll> must have a defined height (e.g., height: 500px or flex: 1 in a flex container). If the height is 0, the virtualizer cannot calculate the visible range.
• [or3-scroll] Duplicate item key detected: Ensure every item in the items array has a unique key (defined by itemKey). Duplicate keys cause rendering artifacts.
• [or3-scroll] estimateHeight must be positive: The estimateHeight prop must be greater than 0.