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/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)}
-		<ChatMessage
-			class="mx-auto w-full max-w-[48rem]"
-			{message}
-			{isLastAssistantMessage}
-			{siblingInfo}
-		/>
+		<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}
@@ -359,57 +356,59 @@
 		onscroll={handleScroll}
 		role="main"
 	>
-		<ChatMessages
-			class="mb-16 md:mb-24"
-			messages={activeMessages()}
-			onUserAction={() => {
-				autoScroll.enable();
-				autoScroll.scrollToBottom();
-			}}
-		/>
+		<div class="flex flex-col">
+			<ChatMessages
+				class="mb-16 md:mb-24"
+				messages={activeMessages()}
+				onUserAction={() => {
+					autoScroll.enable();
+					autoScroll.scrollToBottom();
+				}}
+			/>
 
-		<div
-			class="pointer-events-none sticky right-0 bottom-4 left-0 mt-auto"
-			in:slide={{ duration: 150, axis: 'y' }}
-		>
-			<ChatScreenProcessingInfo />
+			<div
+				class="pointer-events-none sticky right-0 bottom-4 left-0 mt-auto"
+				in:slide={{ duration: 150, axis: 'y' }}
+			>
+				<ChatScreenProcessingInfo />
 
-			{#if hasPropsError}
-				<div
-					class="pointer-events-auto mx-auto mb-4 max-w-[48rem] px-1"
-					in:fly={{ y: 10, duration: 250 }}
-				>
-					<Alert.Root variant="destructive">
-						<AlertTriangle class="h-4 w-4" />
-						<Alert.Title class="flex items-center justify-between">
-							<span>Server unavailable</span>
-							<button
-								onclick={() => serverStore.fetch()}
-								disabled={isServerLoading}
-								class="flex items-center gap-1.5 rounded-lg bg-destructive/20 px-2 py-1 text-xs font-medium hover:bg-destructive/30 disabled:opacity-50"
-							>
-								<RefreshCw class="h-3 w-3 {isServerLoading ? 'animate-spin' : ''}" />
-								{isServerLoading ? 'Retrying...' : 'Retry'}
-							</button>
-						</Alert.Title>
-						<Alert.Description>{serverError()}</Alert.Description>
-					</Alert.Root>
+				{#if hasPropsError}
+					<div
+						class="pointer-events-auto mx-auto mb-4 max-w-[48rem] px-1"
+						in:fly={{ y: 10, duration: 250 }}
+					>
+						<Alert.Root variant="destructive">
+							<AlertTriangle class="h-4 w-4" />
+							<Alert.Title class="flex items-center justify-between">
+								<span>Server unavailable</span>
+								<button
+									onclick={() => serverStore.fetch()}
+									disabled={isServerLoading}
+									class="flex items-center gap-1.5 rounded-lg bg-destructive/20 px-2 py-1 text-xs font-medium hover:bg-destructive/30 disabled:opacity-50"
+								>
+									<RefreshCw class="h-3 w-3 {isServerLoading ? 'animate-spin' : ''}" />
+									{isServerLoading ? 'Retrying...' : 'Retry'}
+								</button>
+							</Alert.Title>
+							<Alert.Description>{serverError()}</Alert.Description>
+						</Alert.Root>
+					</div>
+				{/if}
+
+				<div class="conversation-chat-form pointer-events-auto rounded-t-3xl">
+					<ChatScreenForm
+						disabled={hasPropsError || isEditing()}
+						{initialMessage}
+						isLoading={isCurrentConversationLoading}
+						onFileRemove={handleFileRemove}
+						onFileUpload={handleFileUpload}
+						onSend={handleSendMessage}
+						onStop={() => chatStore.stopGeneration()}
+						onSystemPromptAdd={handleSystemPromptAdd}
+						showHelperText={false}
+						bind:uploadedFiles
+					/>
 				</div>
-			{/if}
-
-			<div class="conversation-chat-form pointer-events-auto rounded-t-3xl">
-				<ChatScreenForm
-					disabled={hasPropsError || isEditing()}
-					{initialMessage}
-					isLoading={isCurrentConversationLoading}
-					onFileRemove={handleFileRemove}
-					onFileUpload={handleFileUpload}
-					onSend={handleSendMessage}
-					onStop={() => chatStore.stopGeneration()}
-					onSystemPromptAdd={handleSystemPromptAdd}
-					showHelperText={false}
-					bind:uploadedFiles
-				/>
 			</div>
 		</div>
 	</div>

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;
 	}