perf: batch scrollToBottom during streaming via rAF (#21946)

src/lib/components/chat/Chat.svelte

@@ -1281,6 +1281,16 @@
 			});
 		}
 	};
+
+	let scrollRAF = null;
+	const scheduleScrollToBottom = () => {
+		if (!scrollRAF) {
+			scrollRAF = requestAnimationFrame(async () => {
+				scrollRAF = null;
+				await scrollToBottom();
+			});
+		}
+	};
 	const chatCompletedHandler = async (_chatId, modelId, responseMessageId, messages) => {
 		const res = await chatCompleted(localStorage.token, {
 			model: modelId,
@@ -1698,7 +1708,7 @@
 		await tick();
 
 		if (autoScroll) {
-			scrollToBottom();
+			scheduleScrollToBottom();
 		}
 	};
 
@@ -2482,7 +2492,7 @@
 					}
 
 					if (autoScroll) {
-						scrollToBottom();
+						scheduleScrollToBottom();
 					}
 				}