webui: Improve Chat Messages initial scroll + auto-scroll logic + add lazy loading with transitions to content blocks (#20999)

* refactor: Always use agentic content renderer for Assistant Message

* feat: Improve initial scroll + auto-scroll logic + implement fade in action for content blocks

* chore: update webui build output

tools/server/webui/src/lib/actions/fade-in-view.svelte.ts

@@ -0,0 +1,54 @@
+/**
+ * Svelte action that fades in an element when it enters the viewport.
+ * Uses IntersectionObserver for efficient viewport detection.
+ *
+ * If skipIfVisible is set and the element is already visible in the viewport
+ * when the action attaches (e.g. a markdown block promoted from unstable
+ * during streaming), the fade is skipped entirely to avoid a flash.
+ */
+export function fadeInView(
+	node: HTMLElement,
+	options: { duration?: number; y?: number; skipIfVisible?: boolean } = {}
+) {
+	const { duration = 300, y = 0, skipIfVisible = false } = options;
+
+	if (skipIfVisible) {
+		const rect = node.getBoundingClientRect();
+		const isAlreadyVisible =
+			rect.top < window.innerHeight &&
+			rect.bottom > 0 &&
+			rect.left < window.innerWidth &&
+			rect.right > 0;
+
+		if (isAlreadyVisible) {
+			return;
+		}
+	}
+
+	node.style.opacity = '0';
+	node.style.transform = `translateY(${y}px)`;
+	node.style.transition = `opacity ${duration}ms ease-out, transform ${duration}ms ease-out`;
+
+	$effect(() => {
+		const observer = new IntersectionObserver(
+			(entries) => {
+				for (const entry of entries) {
+					if (entry.isIntersecting) {
+						requestAnimationFrame(() => {
+							node.style.opacity = '1';
+							node.style.transform = 'translateY(0)';
+						});
+						observer.disconnect();
+					}
+				}
+			},
+			{ threshold: 0.05 }
+		);
+
+		observer.observe(node);
+
+		return () => {
+			observer.disconnect();
+		};
+	});
+}

tools/server/webui/src/lib/components/app/chat/ChatMessages/ChatMessageAssistant.svelte

@@ -3,14 +3,12 @@
 		ChatMessageAgenticContent,
 		ChatMessageActions,
 		ChatMessageStatistics,
-		MarkdownContent,
 		ModelBadge,
 		ModelsSelector
 	} from '$lib/components/app';
 	import { getMessageEditContext } from '$lib/contexts';
 	import { useProcessingState } from '$lib/hooks/use-processing-state.svelte';
 	import { isLoading, isChatStreaming } from '$lib/stores/chat.svelte';
-	import { agenticStreamingToolCall } from '$lib/stores/agentic.svelte';
 	import { autoResizeTextarea, copyToClipboard, isIMEComposing } from '$lib/utils';
 	import { tick } from 'svelte';
 	import { fade } from 'svelte/transition';
@@ -87,13 +85,7 @@
 	const hasAgenticMarkers = $derived(
 		messageContent?.includes(AGENTIC_TAGS.TOOL_CALL_START) ?? false
 	);
-	const hasStreamingToolCall = $derived(
-		isChatStreaming() && agenticStreamingToolCall(message.convId) !== null
-	);
 	const hasReasoningMarkers = $derived(messageContent?.includes(REASONING_TAGS.START) ?? false);
-	const isStructuredContent = $derived(
-		hasAgenticMarkers || hasReasoningMarkers || hasStreamingToolCall
-	);
 	const processingState = useProcessingState();
 
 	let currentConfig = $derived(config());
@@ -256,15 +248,13 @@
 	{:else if message.role === MessageRole.ASSISTANT}
 		{#if showRawOutput}
 			<pre class="raw-output">{messageContent || ''}</pre>
-		{:else if isStructuredContent}
+		{:else}
 			<ChatMessageAgenticContent
 				content={messageContent || ''}
 				isStreaming={isChatStreaming()}
 				highlightTurns={highlightAgenticTurns}
 				{message}
 			/>
-		{:else}
-			<MarkdownContent content={messageContent || ''} attachments={message.extra} />
 		{/if}
 	{:else}
 		<div class="text-sm whitespace-pre-wrap">

tools/server/webui/src/lib/components/app/chat/ChatMessages/ChatMessages.svelte

@@ -1,4 +1,5 @@
 <script lang="ts">
+	import { fadeInView } from '$lib/actions/fade-in-view.svelte';
 	import { ChatMessage } from '$lib/components/app';
 	import { setChatActionsContext } from '$lib/contexts';
 	import { MessageRole } from '$lib/enums';
@@ -140,13 +141,18 @@
 	});
 </script>
 
-<div class="flex h-full flex-col space-y-10 pt-24 {className}" style="height: auto; ">
+<div
+	class="flex h-full flex-col space-y-10 pt-24 {className}"
+	style="height: auto; min-height: calc(100dvh - 14rem);"
+>
 	{#each displayMessages as { message, isLastAssistantMessage, siblingInfo } (message.id)}
+		<div use:fadeInView>
 			<ChatMessage
 				class="mx-auto w-full max-w-[48rem]"
 				{message}
 				{isLastAssistantMessage}
 				{siblingInfo}
 			/>
+		</div>
 	{/each}
 </div>

tools/server/webui/src/lib/components/app/chat/ChatScreen/ChatScreen.svelte

@@ -12,7 +12,6 @@
 	} from '$lib/components/app';
 	import * as Alert from '$lib/components/ui/alert';
 	import * as AlertDialog from '$lib/components/ui/alert-dialog';
-	import { INITIAL_SCROLL_DELAY } from '$lib/constants';
 	import { KeyboardKey } from '$lib/enums';
 	import { createAutoScrollController } from '$lib/hooks/use-auto-scroll.svelte';
 	import {
@@ -48,7 +47,7 @@
 	let showFileErrorDialog = $state(false);
 	let uploadedFiles = $state<ChatUploadedFile[]>([]);
 
-	const autoScroll = createAutoScrollController();
+	const autoScroll = createAutoScrollController({ isColumnReverse: true });
 
 	let fileErrorData = $state<{
 		generallyUnsupported: File[];
@@ -310,13 +309,15 @@
 
 	afterNavigate(() => {
 		if (!disableAutoScroll) {
-			setTimeout(() => autoScroll.scrollToBottom('instant'), INITIAL_SCROLL_DELAY);
+			autoScroll.enable();
 		}
 	});
 
 	onMount(() => {
+		autoScroll.startObserving();
+
 		if (!disableAutoScroll) {
-			setTimeout(() => autoScroll.scrollToBottom('instant'), INITIAL_SCROLL_DELAY);
+			autoScroll.enable();
 		}
 
 		const pendingDraft = chatStore.consumePendingDraft();
@@ -333,10 +334,6 @@
 	$effect(() => {
 		autoScroll.setDisabled(disableAutoScroll);
 	});
-
-	$effect(() => {
-		autoScroll.updateInterval(isCurrentConversationLoading);
-	});
 </script>
 
 {#if isDragOver}
@@ -351,7 +348,7 @@
 	<div
 		bind:this={chatScrollContainer}
 		aria-label="Chat interface with file drop zone"
-		class="flex h-full flex-col overflow-y-auto px-4 md:px-6"
+		class="flex h-full flex-col-reverse overflow-y-auto px-4 md:px-6"
 		ondragenter={handleDragEnter}
 		ondragleave={handleDragLeave}
 		ondragover={handleDragOver}
@@ -359,6 +356,7 @@
 		onscroll={handleScroll}
 		role="main"
 	>
+		<div class="flex flex-col">
 			<ChatMessages
 				class="mb-16 md:mb-24"
 				messages={activeMessages()}
@@ -413,6 +411,7 @@
 				</div>
 			</div>
 		</div>
+	</div>
 {:else if isServerLoading}
 	<!-- Server Loading State -->
 	<ServerLoadingSplash />

tools/server/webui/src/lib/components/app/content/MarkdownContent.svelte

@@ -36,6 +36,7 @@
 	import { createAutoScrollController } from '$lib/hooks/use-auto-scroll.svelte';
 	import type { DatabaseMessageExtra } from '$lib/types/database';
 	import { config } from '$lib/stores/settings.svelte';
+	import { fadeInView } from '$lib/actions/fade-in-view.svelte';
 
 	interface Props {
 		attachments?: DatabaseMessageExtra[];
@@ -598,7 +599,7 @@
 		: ''}"
 >
 	{#each renderedBlocks as block (block.id)}
-		<div class="markdown-block" data-block-id={block.id}>
+		<div class="markdown-block" data-block-id={block.id} use:fadeInView={{ skipIfVisible: true }}>
 			<!-- eslint-disable-next-line no-at-html-tags -->
 			{@html block.html}
 		</div>
@@ -651,7 +652,6 @@
 />
 
 <style>
-	.markdown-block,
 	.markdown-block--unstable {
 		display: contents;
 	}

tools/server/webui/src/lib/constants/auto-scroll.ts

@@ -1,3 +1,2 @@
 export const AUTO_SCROLL_INTERVAL = 100;
-export const INITIAL_SCROLL_DELAY = 50;
 export const AUTO_SCROLL_AT_BOTTOM_THRESHOLD = 10;

tools/server/webui/src/lib/hooks/use-auto-scroll.svelte.ts

@@ -1,8 +1,8 @@
 import { AUTO_SCROLL_AT_BOTTOM_THRESHOLD, AUTO_SCROLL_INTERVAL } from '$lib/constants';
 
 export interface AutoScrollOptions {
-	/** Whether auto-scroll is disabled globally (e.g., from settings) */
 	disabled?: boolean;
+	isColumnReverse?: boolean;
 }
 
 /**
@@ -12,6 +12,7 @@ export interface AutoScrollOptions {
  * - Auto-scrolls to bottom during streaming/loading
  * - Stops auto-scroll when user manually scrolls up
  * - Resumes auto-scroll when user scrolls back to bottom
+ * - Supports both normal and column-reverse scroll containers
  */
 export class AutoScrollController {
 	private _autoScrollEnabled = $state(true);
@@ -21,9 +22,14 @@ export class AutoScrollController {
 	private _scrollTimeout: ReturnType<typeof setTimeout> | undefined;
 	private _container: HTMLElement | undefined;
 	private _disabled: boolean;
+	private _isColumnReverse: boolean;
+	private _mutationObserver: MutationObserver | null = null;
+	private _rafPending = false;
+	private _observerEnabled = false;
 
 	constructor(options: AutoScrollOptions = {}) {
 		this._disabled = options.disabled ?? false;
+		this._isColumnReverse = options.isColumnReverse ?? false;
 	}
 
 	get autoScrollEnabled(): boolean {
@@ -38,7 +44,12 @@ export class AutoScrollController {
 	 * Binds the controller to a scrollable container element.
 	 */
 	setContainer(container: HTMLElement | undefined): void {
+		this._doStopObserving();
 		this._container = container;
+
+		if (this._observerEnabled && container && !this._disabled) {
+			this._doStartObserving();
+		}
 	}
 
 	/**
@@ -49,6 +60,9 @@ export class AutoScrollController {
 		if (disabled) {
 			this._autoScrollEnabled = false;
 			this.stopInterval();
+			this._doStopObserving();
+		} else if (this._observerEnabled && this._container && !this._mutationObserver) {
+			this._doStartObserving();
 		}
 	}
 
@@ -59,10 +73,23 @@ export class AutoScrollController {
 		if (this._disabled || !this._container) return;
 
 		const { scrollTop, scrollHeight, clientHeight } = this._container;
-		const distanceFromBottom = scrollHeight - scrollTop - clientHeight;
+
+		let distanceFromBottom: number;
+		let isScrollingUp: boolean;
+
+		if (this._isColumnReverse) {
+			// column-reverse: scrollTop=0 at bottom, negative when scrolled up
+			distanceFromBottom = Math.abs(scrollTop);
+			isScrollingUp = scrollTop < this._lastScrollTop;
+		} else {
+			// normal: scrollTop=0 at top, increases when scrolled down
+			distanceFromBottom = scrollHeight - clientHeight - scrollTop;
+			isScrollingUp = scrollTop < this._lastScrollTop;
+		}
+
 		const isAtBottom = distanceFromBottom < AUTO_SCROLL_AT_BOTTOM_THRESHOLD;
 
-		if (scrollTop < this._lastScrollTop && !isAtBottom) {
+		if (isScrollingUp && !isAtBottom) {
 			this._userScrolledUp = true;
 			this._autoScrollEnabled = false;
 		} else if (isAtBottom && this._userScrolledUp) {
@@ -90,10 +117,12 @@ export class AutoScrollController {
 	scrollToBottom(behavior: ScrollBehavior = 'smooth'): void {
 		if (this._disabled || !this._container) return;
 
-		this._container.scrollTo({
+		if (this._isColumnReverse) {
-			top: this._container.scrollHeight,
+			// column-reverse: scrollTop=0 is the bottom
-			behavior
+			this._container.scrollTo({ top: 0, behavior });
-		});
+		} else {
+			this._container.scrollTo({ top: this._container.scrollHeight, behavior });
+		}
 	}
 
 	/**
@@ -150,11 +179,69 @@ export class AutoScrollController {
 	 */
 	destroy(): void {
 		this.stopInterval();
+		this._doStopObserving();
+
 		if (this._scrollTimeout) {
 			clearTimeout(this._scrollTimeout);
 			this._scrollTimeout = undefined;
 		}
 	}
+
+	/**
+	 * Starts a MutationObserver on the container that auto-scrolls to bottom
+	 * on content changes. More responsive than interval-based polling.
+	 */
+	startObserving(): void {
+		this._observerEnabled = true;
+
+		if (this._container && !this._disabled && !this._mutationObserver) {
+			this._doStartObserving();
+		}
+	}
+
+	/**
+	 * Stops the MutationObserver.
+	 */
+	stopObserving(): void {
+		this._observerEnabled = false;
+		this._doStopObserving();
+	}
+
+	private _doStartObserving(): void {
+		if (!this._container || this._mutationObserver) return;
+
+		const isReverse = this._isColumnReverse;
+
+		this._mutationObserver = new MutationObserver(() => {
+			if (!this._autoScrollEnabled || this._rafPending) return;
+			this._rafPending = true;
+			requestAnimationFrame(() => {
+				this._rafPending = false;
+				if (this._autoScrollEnabled && this._container) {
+					if (isReverse) {
+						// column-reverse: scrollTop=0 is the bottom
+						this._container.scrollTop = 0;
+					} else {
+						this._container.scrollTop = this._container.scrollHeight;
+					}
+				}
+			});
+		});
+
+		this._mutationObserver.observe(this._container, {
+			childList: true,
+			subtree: true,
+			characterData: true
+		});
+	}
+
+	private _doStopObserving(): void {
+		if (this._mutationObserver) {
+			this._mutationObserver.disconnect();
+			this._mutationObserver = null;
+		}
+		this._rafPending = false;
+	}
 }
 
 /**