fix(core): ai chat scrolldown indicator (#13382)

Close [AI-401](https://linear.app/affine-design/issue/AI-401)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Refactor**
* Improved scrolling behavior in the AI chat messages panel by making
the entire panel the scroll container, resulting in more consistent
scroll handling.
* Adjusted the position of the down-indicator for better visibility
during scrolling.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/frontend/core/src/blocksuite/ai/components/ai-chat-content/ai-chat-content.ts

@@ -314,9 +314,8 @@ export class AIChatContent extends SignalWatcher(
     if (chatMessages) {
       chatMessages.updateComplete
         .then(() => {
-          const scrollContainer = chatMessages.getScrollContainer();
-          scrollContainer?.addEventListener('scrollend', () => {
-            this.lastScrollTop = scrollContainer.scrollTop;
+          chatMessages.addEventListener('scrollend', () => {
+            this.lastScrollTop = chatMessages.scrollTop;
           });
           this._scrollListenersInitialized = true;
         })

packages/frontend/core/src/blocksuite/ai/components/ai-chat-messages/ai-chat-messages.ts

@@ -13,7 +13,7 @@ import type { BaseSelection, ExtensionType } from '@blocksuite/affine/store';
 import { ArrowDownBigIcon as ArrowDownIcon } from '@blocksuite/icons/lit';
 import type { Signal } from '@preact/signals-core';
 import { css, html, nothing, type PropertyValues } from 'lit';
-import { property, query, state } from 'lit/decorators.js';
+import { property, state } from 'lit/decorators.js';
 import { classMap } from 'lit/directives/class-map.js';
 import { repeat } from 'lit/directives/repeat.js';
 import { debounce } from 'lodash-es';
@@ -125,10 +125,10 @@ export class AIChatMessages extends WithDisposable(ShadowlessElement) {
     }
 
     .down-indicator {
-      position: absolute;
+      position: fixed;
       left: 50%;
       transform: translate(-50%, 0);
-      bottom: 24px;
+      bottom: 166px;
       z-index: 1;
       border-radius: 50%;
       width: 32px;
@@ -217,9 +217,6 @@ export class AIChatMessages extends WithDisposable(ShadowlessElement) {
   @property({ attribute: false })
   accessor onOpenDoc!: (docId: string, sessionId?: string) => void;
 
-  @query('.chat-panel-messages-container')
-  accessor messagesContainer: HTMLDivElement | null = null;
-
   @property({
     type: String,
     attribute: 'data-testid',
@@ -227,10 +224,6 @@ export class AIChatMessages extends WithDisposable(ShadowlessElement) {
   })
   accessor testId = 'chat-panel-messages';
 
-  getScrollContainer(): HTMLDivElement | null {
-    return this.messagesContainer;
-  }
-
   private get _isNetworkActive() {
     return (
       !!this.networkSearchConfig.visible.value &&
@@ -264,8 +257,7 @@ export class AIChatMessages extends WithDisposable(ShadowlessElement) {
   }
 
   private readonly _onScroll = () => {
-    if (!this.messagesContainer) return;
-    const { clientHeight, scrollTop, scrollHeight } = this.messagesContainer;
+    const { clientHeight, scrollTop, scrollHeight } = this;
     this.canScrollDown = scrollHeight - scrollTop - clientHeight > 200;
   };
 
@@ -293,7 +285,6 @@ export class AIChatMessages extends WithDisposable(ShadowlessElement) {
           'independent-mode': !!this.independentMode,
         })}
         data-testid="chat-panel-messages-container"
-        @scroll=${() => this._debouncedOnScroll()}
       >
         ${filteredItems.length === 0
           ? html`<div
@@ -411,6 +402,12 @@ export class AIChatMessages extends WithDisposable(ShadowlessElement) {
         )
       );
     }
+
+    // Add scroll event listener to the host element
+    this.addEventListener('scroll', this._debouncedOnScroll);
+    disposables.add(() => {
+      this.removeEventListener('scroll', this._debouncedOnScroll);
+    });
   }
 
   protected override updated(_changedProperties: PropertyValues) {
@@ -428,9 +425,8 @@ export class AIChatMessages extends WithDisposable(ShadowlessElement) {
 
   scrollToEnd() {
     requestAnimationFrame(() => {
-      if (!this.messagesContainer) return;
-      this.messagesContainer.scrollTo({
-        top: this.messagesContainer.scrollHeight,
+      this.scrollTo({
+        top: this.scrollHeight,
         behavior: 'smooth',
       });
     });
@@ -438,8 +434,7 @@ export class AIChatMessages extends WithDisposable(ShadowlessElement) {
 
   scrollToPos(top: number) {
     requestAnimationFrame(() => {
-      if (!this.messagesContainer) return;
-      this.messagesContainer.scrollTo({ top });
+      this.scrollTo({ top });
     });
   }