feat: add scroll-to-bottom button to chat + prevent forced scroll down (#23270)

tools/ui/src/lib/components/app/chat/ChatScreen/ChatScreen.svelte

@@ -8,6 +8,7 @@
 		ChatMessages,
 		ChatScreenDragOverlay,
 		ChatScreenProcessingInfo,
+		ChatScreenActionScrollDown,
 		DialogEmptyFileAlert,
 		DialogFileUploadError,
 		DialogChatError,
@@ -338,7 +339,9 @@
 	});
 
 	function handleMessagesReady() {
-		if (!disableAutoScroll && !autoScroll.userScrolledUp) {
+		if (disableAutoScroll) return;
+
+		if (!autoScroll.userScrolledUp) {
 			requestAnimationFrame(() => {
 				autoScroll.scrollToBottom('instant');
 			});
@@ -405,7 +408,7 @@
 			<div
 				class="pointer-events-none {isEmpty
 					? 'absolute bottom-[calc(50dvh-7rem)]'
-					: 'sticky bottom-4'} right-4 left-4 mt-auto pt-16 transition-all duration-200"
+					: 'sticky bottom-4'} right-4 left-4 mt-auto -mb-14 pt-16 transition-all duration-200"
 			>
 				{#if isEmpty}
 					<div class="mb-8 px-4 text-center" use:fadeInView={{ duration: 300 }}>
@@ -419,6 +422,8 @@
 					</div>
 				{/if}
 
+				<ChatScreenActionScrollDown container={chatScrollContainer} />
+
 				{#if page.params.id}
 					<ChatScreenProcessingInfo />
 				{/if}

tools/ui/src/lib/components/app/chat/ChatScreen/ChatScreenActionScrollDown.svelte

@@ -0,0 +1,48 @@
+<script lang="ts">
+	import { ArrowDown } from '@lucide/svelte';
+	import { Button } from '$lib/components/ui/button';
+
+	let { container }: { container: HTMLDivElement | undefined } = $props();
+
+	let show = $state(false);
+
+	function checkVisibility() {
+		if (!container) return;
+		const { scrollTop, scrollHeight, clientHeight } = container;
+		const distanceFromBottom = scrollHeight - clientHeight - scrollTop;
+		show = distanceFromBottom > clientHeight * 0.5;
+	}
+
+	function scrollToBottom() {
+		if (container) {
+			container.scrollTo({
+				top: container.scrollHeight,
+				behavior: 'smooth'
+			});
+		}
+	}
+
+	$effect(() => {
+		const c = container;
+		if (c) {
+			c.addEventListener('scroll', checkVisibility);
+			checkVisibility();
+			return () => {
+				c.removeEventListener('scroll', checkVisibility);
+			};
+		}
+	});
+</script>
+
+<div class="pointer-events-auto relative z-50 mx-auto mb-4 flex max-w-[48rem] justify-center">
+	<Button
+		onclick={scrollToBottom}
+		variant="secondary"
+		size="icon"
+		class="h-10 w-10 rounded-full bg-background/80 shadow-lg backdrop-blur-sm transition-all duration-200 hover:bg-muted/80"
+		aria-label="Scroll to bottom"
+		style="transform: translateY({show ? '0' : '20px'}); opacity: {show ? 1 : 0};"
+	>
+		<ArrowDown class="h-4 w-4" />
+	</Button>
+</div>

tools/ui/src/lib/components/app/chat/index.ts

@@ -667,3 +667,10 @@ export { default as ChatScreenForm } from './ChatScreen/ChatScreenForm.svelte';
  * Only visible when `isCurrentConversationLoading` is true.
  */
 export { default as ChatScreenProcessingInfo } from './ChatScreen/ChatScreenProcessingInfo.svelte';
+
+/**
+ * Scroll-to-bottom action button. Displays a floating button when the user
+ * has scrolled up more than half a viewport height from the bottom.
+ * Takes the chat container element as a prop to manage scroll state internally.
+ */
+export { default as ChatScreenActionScrollDown } from './ChatScreen/ChatScreenActionScrollDown.svelte';

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

@@ -100,6 +100,14 @@ export class AutoScrollController {
 		this._autoScrollEnabled = true;
 	}
 
+	/**
+	 * Resets scroll state when switching conversations.
+	 */
+	resetScrollState(): void {
+		this._userScrolledUp = false;
+		this._autoScrollEnabled = true;
+	}
+
 	/**
 	 * Starts the auto-scroll interval for continuous scrolling during streaming.
 	 */