fix(core): downward arrow is visible when message panel is not scrollable (#9670)

Fix issue [AF-2056](https://linear.app/affine-design/issue/AF-2056).

### What changed?
- Check both `showDownIndicator` and `filteredItems.length` when rendering `DownArrowIcon`.
- Use `scrollIntoView` instead of `scrollToEnd` during AI result transmission.

![image.png](https://graphite-user-uploaded-assets-prod.s3.amazonaws.com/sJGviKxfE3Ap685cl5bj/9810856c-04fe-4381-8a9b-a87a0d8931d6.png)

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

@@ -14,7 +14,7 @@ import { css, html, nothing } from 'lit';
 import { property, query, state } from 'lit/decorators.js';
 import { repeat } from 'lit/directives/repeat.js';
 import { styleMap } from 'lit/directives/style-map.js';
-import { debounce } from 'lodash-es';
+import { debounce, throttle } from 'lodash-es';
 
 import {
   EdgelessEditorActions,
@@ -132,6 +132,9 @@ export class ChatPanelMessages extends WithDisposable(ShadowlessElement) {
   @query('.chat-panel-messages')
   accessor messagesContainer: HTMLDivElement | null = null;
 
+  @query('.message:nth-last-child(2)')
+  accessor lastMessage: HTMLDivElement | null = null;
+
   private _renderAIOnboarding() {
     return this.isLoading ||
       !this.host?.doc.get(FeatureFlagService).getFlag('enable_ai_onboarding')
@@ -189,6 +192,16 @@ export class ChatPanelMessages extends WithDisposable(ShadowlessElement) {
     100
   );
 
+  private readonly _scrollIntoView = () => {
+    if (!this.lastMessage) return;
+    this.lastMessage.scrollIntoView({ behavior: 'smooth' });
+  };
+
+  private readonly _throttledScrollIntoView = throttle(
+    this._scrollIntoView,
+    500
+  );
+
   protected override render() {
     const { items } = this.chatContextValue;
     const { isLoading } = this;
@@ -243,7 +256,7 @@ export class ChatPanelMessages extends WithDisposable(ShadowlessElement) {
               }
             )}
       </div>
-      ${this.showDownIndicator
+      ${this.showDownIndicator && filteredItems.length > 1
         ? html`<div class="down-indicator" @click=${this.scrollToEnd}>
             ${DownArrowIcon}
           </div>`
@@ -287,6 +300,12 @@ export class ChatPanelMessages extends WithDisposable(ShadowlessElement) {
     );
   }
 
+  protected override updated() {
+    if (this.chatContextValue.status === 'transmitting') {
+      this._throttledScrollIntoView();
+    }
+  }
+
   renderItem(item: ChatItem, isLast: boolean) {
     const { status, error } = this.chatContextValue;
     const { host } = this;

packages/frontend/core/src/blocksuite/presets/ai/chat-panel/index.ts

@@ -205,17 +205,11 @@ export class ChatPanel extends WithDisposable(ShadowlessElement) {
     if (
       !this.isLoading &&
       _changedProperties.has('chatContextValue') &&
-      this.chatContextValue.status !== 'idle'
-    ) {
-      if (this.chatContextValue.status === 'transmitting') {
-        this._scrollToEnd();
-      } else if (
-        this.chatContextValue.status === 'loading' ||
+      (this.chatContextValue.status === 'loading' ||
         this.chatContextValue.status === 'error' ||
-        this.chatContextValue.status === 'success'
-      ) {
-        setTimeout(this._scrollToEnd, 500);
-      }
+        this.chatContextValue.status === 'success')
+    ) {
+      setTimeout(this._scrollToEnd, 500);
     }
   }