Refactor chat auto-scroll behavior

web/frontend/src/components/chat/chat-page.tsx

@@ -1,6 +1,6 @@
 import { IconPlus } from "@tabler/icons-react"
 import { useAtom } from "jotai"
-import { type ChangeEvent, useEffect, useRef, useState } from "react"
+import { type ChangeEvent, useRef, useState } from "react"
 import { useTranslation } from "react-i18next"
 import { toast } from "sonner"
 
@@ -18,6 +18,7 @@ import { UserMessage } from "@/components/chat/user-message"
 import { PageHeader } from "@/components/page-header"
 import { Button } from "@/components/ui/button"
 import { Switch } from "@/components/ui/switch"
+import { useChatAutoScroll } from "@/hooks/use-chat-auto-scroll"
 import { useChatModels } from "@/hooks/use-chat-models"
 import { useGateway } from "@/hooks/use-gateway"
 import { usePicoChat } from "@/hooks/use-pico-chat"
@@ -108,10 +109,7 @@ function resolveChatInputDisabledReason({
 
 export function ChatPage() {
   const { t } = useTranslation()
-  const scrollRef = useRef<HTMLDivElement>(null)
   const fileInputRef = useRef<HTMLInputElement>(null)
-  const [isAtBottom, setIsAtBottom] = useState(true)
-  const [hasScrolled, setHasScrolled] = useState(false)
   const [input, setInput] = useState("")
   const [attachments, setAttachments] = useState<ChatAttachment[]>([])
   const [showAssistantDetails, setShowAssistantDetails] = useAtom(
@@ -177,25 +175,14 @@ export function ChatPage() {
       (appConfig?.agents as { defaults?: { streaming_enabled?: boolean } })
         ?.defaults?.streaming_enabled
     ) !== false
-
-  const syncScrollState = (element: HTMLDivElement) => {
-    const { clientHeight, scrollHeight, scrollTop } = element
-    setHasScrolled(scrollTop > 0)
-    setIsAtBottom(scrollHeight - scrollTop <= clientHeight + 10)
-  }
-
-  const handleScroll = (e: React.UIEvent<HTMLDivElement>) => {
-    syncScrollState(e.currentTarget)
-  }
-
-  useEffect(() => {
-    if (scrollRef.current) {
-      if (isAtBottom) {
-        scrollRef.current.scrollTop = scrollRef.current.scrollHeight
-      }
-      syncScrollState(scrollRef.current)
-    }
-  }, [messages, isTyping, isAtBottom])
+  const hasStreamingMessage =
+    streamingEnabled &&
+    messages.some((message) => message.role === "assistant" && message.streaming)
+  const { scrollRef, hasScrolled, handleScroll, handleManualScrollIntent } =
+    useChatAutoScroll({
+      deps: [messages, isTyping],
+      streaming: hasStreamingMessage,
+    })
 
   const handleSend = () => {
     if ((!input.trim() && attachments.length === 0) || !canInput) return
@@ -332,6 +319,8 @@ export function ChatPage() {
       <div
         ref={scrollRef}
         onScroll={handleScroll}
+        onWheelCapture={handleManualScrollIntent}
+        onTouchStart={handleManualScrollIntent}
         className="min-h-0 flex-1 overflow-y-auto px-4 py-6 [scrollbar-gutter:stable] md:px-8 lg:px-24 xl:px-48"
       >
         <div className="mx-auto flex w-full max-w-250 flex-col gap-8 pb-8">

web/frontend/src/hooks/use-chat-auto-scroll.ts

@@ -0,0 +1,168 @@
+import {
+  type UIEvent,
+  useCallback,
+  useEffect,
+  useLayoutEffect,
+  useRef,
+  useState,
+} from "react"
+
+const BOTTOM_FOLLOW_THRESHOLD = 32
+
+function getBottomDistance(element: HTMLDivElement) {
+  return element.scrollHeight - element.scrollTop - element.clientHeight
+}
+
+function getBottomScrollTop(element: HTMLDivElement) {
+  return Math.max(0, element.scrollHeight - element.clientHeight)
+}
+
+function nextScrollStep(distance: number, streaming: boolean) {
+  const magnitude = Math.abs(distance)
+  const factor = streaming ? 0.34 : 0.22
+  const min = streaming ? 12 : 10
+  const max = streaming ? 72 : 56
+  return Math.sign(distance) * Math.min(max, Math.max(min, magnitude * factor))
+}
+
+interface UseChatAutoScrollOptions {
+  deps: readonly unknown[]
+  streaming: boolean
+}
+
+export function useChatAutoScroll({
+  deps,
+  streaming,
+}: UseChatAutoScrollOptions) {
+  const scrollRef = useRef<HTMLDivElement>(null)
+  const stickyRef = useRef(true)
+  const targetScrollTopRef = useRef(0)
+  const animationFrameRef = useRef<number | null>(null)
+  const programmaticScrollRef = useRef(false)
+
+  const [isAtBottom, setIsAtBottom] = useState(true)
+  const [hasScrolled, setHasScrolled] = useState(false)
+
+  const cancelAnimation = useCallback(() => {
+    if (animationFrameRef.current !== null) {
+      window.cancelAnimationFrame(animationFrameRef.current)
+      animationFrameRef.current = null
+    }
+    programmaticScrollRef.current = false
+  }, [])
+
+  const syncStateFromElement = useCallback(
+    (element: HTMLDivElement, options?: { programmatic?: boolean }) => {
+      const distanceToBottom = getBottomDistance(element)
+      setHasScrolled(element.scrollTop > 0)
+
+      if (options?.programmatic && stickyRef.current) {
+        setIsAtBottom(true)
+        return
+      }
+
+      const nextIsAtBottom = distanceToBottom <= BOTTOM_FOLLOW_THRESHOLD
+      stickyRef.current = nextIsAtBottom
+      setIsAtBottom(nextIsAtBottom)
+      if (!nextIsAtBottom) {
+        cancelAnimation()
+      }
+    },
+    [cancelAnimation],
+  )
+
+  const animateToBottom = useCallback(() => {
+    const element = scrollRef.current
+    if (!element || !stickyRef.current) {
+      return
+    }
+
+    targetScrollTopRef.current = getBottomScrollTop(element)
+
+    if (animationFrameRef.current !== null) {
+      return
+    }
+
+    const step = () => {
+      const currentElement = scrollRef.current
+      if (!currentElement || !stickyRef.current) {
+        cancelAnimation()
+        return
+      }
+
+      const target = Math.max(
+        targetScrollTopRef.current,
+        getBottomScrollTop(currentElement),
+      )
+      targetScrollTopRef.current = target
+
+      const distance = target - currentElement.scrollTop
+      if (Math.abs(distance) <= 1) {
+        currentElement.scrollTop = target
+        animationFrameRef.current = null
+        programmaticScrollRef.current = false
+        syncStateFromElement(currentElement, { programmatic: true })
+        return
+      }
+
+      programmaticScrollRef.current = true
+      currentElement.scrollTop += nextScrollStep(distance, streaming)
+      syncStateFromElement(currentElement, { programmatic: true })
+      animationFrameRef.current = window.requestAnimationFrame(step)
+    }
+
+    animationFrameRef.current = window.requestAnimationFrame(step)
+  }, [cancelAnimation, streaming, syncStateFromElement])
+
+  const handleScroll = useCallback(
+    (event: UIEvent<HTMLDivElement>) => {
+      const element = event.currentTarget
+      const isProgrammatic = programmaticScrollRef.current
+      if (!isProgrammatic) {
+        const nextIsAtBottom =
+          getBottomDistance(element) <= BOTTOM_FOLLOW_THRESHOLD
+        stickyRef.current = nextIsAtBottom
+      }
+      syncStateFromElement(element, { programmatic: isProgrammatic })
+    },
+    [syncStateFromElement],
+  )
+
+  const handleManualScrollIntent = useCallback(() => {
+    if (!stickyRef.current) {
+      return
+    }
+    stickyRef.current = false
+    setIsAtBottom(false)
+    cancelAnimation()
+  }, [cancelAnimation])
+
+  useLayoutEffect(() => {
+    const element = scrollRef.current
+    if (!element) {
+      return
+    }
+
+    if (!hasScrolled && element.scrollHeight > 0) {
+      element.scrollTop = getBottomScrollTop(element)
+      stickyRef.current = true
+      setIsAtBottom(true)
+      setHasScrolled(element.scrollTop > 0)
+      return
+    }
+
+    if (stickyRef.current) {
+      animateToBottom()
+    }
+  }, [animateToBottom, hasScrolled, ...deps])
+
+  useEffect(() => cancelAnimation, [cancelAnimation])
+
+  return {
+    scrollRef,
+    isAtBottom,
+    hasScrolled,
+    handleScroll,
+    handleManualScrollIntent,
+  }
+}