revert(app): "STUPID SEXY TIMELINE (#16420)" (#16745)

3 files changed, 58 insertions, 198 deletions

diff --git a/packages/ui/src/hooks/create-auto-scroll.tsx b/packages/ui/src/hooks/create-auto-scroll.tsx
index d36102590..3dc520c62 100644
--- a/packages/ui/src/hooks/create-auto-scroll.tsx
+++ b/packages/ui/src/hooks/create-auto-scroll.tsx
@@ -1,8 +1,6 @@
 import { createEffect, on, onCleanup } from "solid-js"
 import { createStore } from "solid-js/store"
 import { createResizeObserver } from "@solid-primitives/resize-observer"
-import { animate, type AnimationPlaybackControls } from "motion"
-import { FAST_SPRING } from "../components/motion"
 
 export interface AutoScrollOptions {
   working: () => boolean
@@ -11,28 +9,13 @@ export interface AutoScrollOptions {
   bottomThreshold?: number
 }
 
-const SETTLE_MS = 500
-const AUTO_SCROLL_GRACE_MS = 120
-const AUTO_SCROLL_EPSILON = 0.5
-const MANUAL_ANCHOR_MS = 3000
-const MANUAL_ANCHOR_QUIET_FRAMES = 24
-
 export function createAutoScroll(options: AutoScrollOptions) {
   let scroll: HTMLElement | undefined
   let settling = false
   let settleTimer: ReturnType<typeof setTimeout> | undefined
+  let autoTimer: ReturnType<typeof setTimeout> | undefined
   let cleanup: (() => void) | undefined
-  let programmaticUntil = 0
-  let scrollAnim: AnimationPlaybackControls | undefined
-  let hold:
-    | {
-        el: HTMLElement
-        top: number
-        until: number
-        quiet: number
-        frame: number | undefined
-      }
-    | undefined
+  let auto: { top: number; time: number } | undefined
 
   const threshold = () => options.bottomThreshold ?? 10
 
@@ -44,160 +27,77 @@ export function createAutoScroll(options: AutoScrollOptions) {
   const active = () => options.working() || settling
 
   const distanceFromBottom = (el: HTMLElement) => {
-    // With column-reverse, scrollTop=0 is at the bottom, negative = scrolled up
-    return Math.abs(el.scrollTop)
+    return el.scrollHeight - el.clientHeight - el.scrollTop
   }
 
   const canScroll = (el: HTMLElement) => {
     return el.scrollHeight - el.clientHeight > 1
   }
 
-  const markProgrammatic = () => {
-    programmaticUntil = Date.now() + AUTO_SCROLL_GRACE_MS
-  }
+  // Browsers can dispatch scroll events asynchronously. If new content arrives
+  // between us calling `scrollTo()` and the subsequent `scroll` event firing,
+  // the handler can see a non-zero `distanceFromBottom` and incorrectly assume
+  // the user scrolled.
+  const markAuto = (el: HTMLElement) => {
+    auto = {
+      top: Math.max(0, el.scrollHeight - el.clientHeight),
+      time: Date.now(),
+    }
 
-  const clearHold = () => {
-    const next = hold
-    if (!next) return
-    if (next.frame !== undefined) cancelAnimationFrame(next.frame)
-    hold = undefined
+    if (autoTimer) clearTimeout(autoTimer)
+    autoTimer = setTimeout(() => {
+      auto = undefined
+      autoTimer = undefined
+    }, 1500)
   }
 
-  const tickHold = () => {
-    const next = hold
-    const el = scroll
-    if (!next || !el) return false
-    if (Date.now() > next.until) {
-      clearHold()
-      return false
-    }
-    if (!next.el.isConnected) {
-      clearHold()
-      return false
-    }
+  const isAuto = (el: HTMLElement) => {
+    const a = auto
+    if (!a) return false
 
-    const current = next.el.getBoundingClientRect().top
-    if (!Number.isFinite(current)) {
-      clearHold()
+    if (Date.now() - a.time > 1500) {
+      auto = undefined
       return false
     }
 
-    const delta = current - next.top
-    if (Math.abs(delta) <= AUTO_SCROLL_EPSILON) {
-      next.quiet += 1
-      if (next.quiet > MANUAL_ANCHOR_QUIET_FRAMES) {
-        clearHold()
-        return false
-      }
-      return true
-    }
-
-    next.quiet = 0
-    if (!store.userScrolled) {
-      setStore("userScrolled", true)
-      options.onUserInteracted?.()
-    }
-    el.scrollTop += delta
-    markProgrammatic()
-    return true
-  }
-
-  const scheduleHold = () => {
-    const next = hold
-    if (!next) return
-    if (next.frame !== undefined) return
-
-    next.frame = requestAnimationFrame(() => {
-      const value = hold
-      if (!value) return
-      value.frame = undefined
-      if (!tickHold()) return
-      scheduleHold()
-    })
+    return Math.abs(el.scrollTop - a.top) < 2
   }
 
-  const preserve = (target: HTMLElement) => {
+  const scrollToBottomNow = (behavior: ScrollBehavior) => {
     const el = scroll
     if (!el) return
-
-    if (!store.userScrolled) {
-      setStore("userScrolled", true)
-      options.onUserInteracted?.()
+    markAuto(el)
+    if (behavior === "smooth") {
+      el.scrollTo({ top: el.scrollHeight, behavior })
+      return
     }
 
-    const top = target.getBoundingClientRect().top
-    if (!Number.isFinite(top)) return
-
-    clearHold()
-    hold = {
-      el: target,
-      top,
-      until: Date.now() + MANUAL_ANCHOR_MS,
-      quiet: 0,
-      frame: undefined,
-    }
-    scheduleHold()
+    // `scrollTop` assignment bypasses any CSS `scroll-behavior: smooth`.
+    el.scrollTop = el.scrollHeight
   }
 
   const scrollToBottom = (force: boolean) => {
     if (!force && !active()) return
 
-    clearHold()
-
     if (force && store.userScrolled) setStore("userScrolled", false)
 
     const el = scroll
     if (!el) return
 
-    if (scrollAnim) cancelSmooth()
     if (!force && store.userScrolled) return
 
-    // With column-reverse, scrollTop=0 is at the bottom
-    if (Math.abs(el.scrollTop) <= AUTO_SCROLL_EPSILON) {
-      markProgrammatic()
+    const distance = distanceFromBottom(el)
+    if (distance < 2) {
+      markAuto(el)
       return
     }
 
-    el.scrollTop = 0
-    markProgrammatic()
-  }
-
-  const cancelSmooth = () => {
-    if (scrollAnim) {
-      scrollAnim.stop()
-      scrollAnim = undefined
-    }
+    // For auto-following content we prefer immediate updates to avoid
+    // visible "catch up" animations while content is still settling.
+    scrollToBottomNow("auto")
   }
 
-  const smoothScrollToBottom = () => {
-    const el = scroll
-    if (!el) return
-
-    cancelSmooth()
-    if (store.userScrolled) setStore("userScrolled", false)
-
-    // With column-reverse, scrollTop=0 is at the bottom
-    if (Math.abs(el.scrollTop) <= AUTO_SCROLL_EPSILON) {
-      markProgrammatic()
-      return
-    }
-
-    scrollAnim = animate(el.scrollTop, 0, {
-      ...FAST_SPRING,
-      onUpdate: (v) => {
-        markProgrammatic()
-        el.scrollTop = v
-      },
-      onComplete: () => {
-        scrollAnim = undefined
-        markProgrammatic()
-      },
-    })
-  }
-
-  const stop = (input?: { hold?: boolean }) => {
-    if (input?.hold !== false) clearHold()
-
+  const stop = () => {
     const el = scroll
     if (!el) return
     if (!canScroll(el)) {
@@ -206,25 +106,15 @@ export function createAutoScroll(options: AutoScrollOptions) {
     }
     if (store.userScrolled) return
 
-    markProgrammatic()
     setStore("userScrolled", true)
     options.onUserInteracted?.()
   }
 
   const handleWheel = (e: WheelEvent) => {
-    if (e.deltaY !== 0) clearHold()
-
-    if (e.deltaY > 0) {
-      const el = scroll
-      if (!el) return
-      if (distanceFromBottom(el) >= threshold()) return
-      if (store.userScrolled) setStore("userScrolled", false)
-      markProgrammatic()
-      return
-    }
-
     if (e.deltaY >= 0) return
-    cancelSmooth()
+    // If the user is scrolling within a nested scrollable region (tool output,
+    // code block, etc), don't treat it as leaving the "follow bottom" mode.
+    // Those regions opt in via `data-scrollable`.
     const el = scroll
     const target = e.target instanceof Element ? e.target : undefined
     const nested = target?.closest("[data-scrollable]")
@@ -236,27 +126,23 @@ export function createAutoScroll(options: AutoScrollOptions) {
     const el = scroll
     if (!el) return
 
-    if (hold) {
-      if (Date.now() < programmaticUntil) return
-      clearHold()
-    }
-
     if (!canScroll(el)) {
       if (store.userScrolled) setStore("userScrolled", false)
-      markProgrammatic()
       return
     }
 
     if (distanceFromBottom(el) < threshold()) {
-      if (Date.now() < programmaticUntil) return
       if (store.userScrolled) setStore("userScrolled", false)
-      markProgrammatic()
       return
     }
 
-    if (!store.userScrolled && Date.now() < programmaticUntil) return
+    // Ignore scroll events triggered by our own scrollToBottom calls.
+    if (!store.userScrolled && isAuto(el)) {
+      scrollToBottom(false)
+      return
+    }
 
-    stop({ hold: false })
+    stop()
   }
 
   const handleInteraction = () => {
@@ -268,11 +154,6 @@ export function createAutoScroll(options: AutoScrollOptions) {
   }
 
   const updateOverflowAnchor = (el: HTMLElement) => {
-    if (hold) {
-      el.style.overflowAnchor = "none"
-      return
-    }
-
     const mode = options.overflowAnchor ?? "dynamic"
 
     if (mode === "none") {
@@ -292,17 +173,15 @@ export function createAutoScroll(options: AutoScrollOptions) {
     () => store.contentRef,
     () => {
       const el = scroll
-      if (hold) {
-        scheduleHold()
-        return
-      }
       if (el && !canScroll(el)) {
         if (store.userScrolled) setStore("userScrolled", false)
-        markProgrammatic()
         return
       }
       if (!active()) return
       if (store.userScrolled) return
+      // ResizeObserver fires after layout, before paint.
+      // Keep the bottom locked in the same frame to avoid visible
+      // "jump up then catch up" artifacts while streaming content.
       scrollToBottom(false)
     },
   )
@@ -321,11 +200,13 @@ export function createAutoScroll(options: AutoScrollOptions) {
       settling = true
       settleTimer = setTimeout(() => {
         settling = false
-      }, SETTLE_MS)
+      }, 300)
     }),
   )
 
   createEffect(() => {
+    // Track `userScrolled` even before `scrollRef` is attached, so we can
+    // update overflow anchoring once the element exists.
     store.userScrolled
     const el = scroll
     if (!el) return
@@ -334,8 +215,7 @@ export function createAutoScroll(options: AutoScrollOptions) {
 
   onCleanup(() => {
     if (settleTimer) clearTimeout(settleTimer)
-    clearHold()
-    cancelSmooth()
+    if (autoTimer) clearTimeout(autoTimer)
     if (cleanup) cleanup()
   })
 
@@ -348,12 +228,8 @@ export function createAutoScroll(options: AutoScrollOptions) {
 
       scroll = el
 
-      if (!el) {
-        clearHold()
-        return
-      }
+      if (!el) return
 
-      markProgrammatic()
       updateOverflowAnchor(el)
       el.addEventListener("wheel", handleWheel, { passive: true })
 
@@ -364,18 +240,13 @@ export function createAutoScroll(options: AutoScrollOptions) {
     contentRef: (el: HTMLElement | undefined) => setStore("contentRef", el),
     handleScroll,
     handleInteraction,
-    preserve,
     pause: stop,
-    forceScrollToBottom: () => scrollToBottom(true),
-    smoothScrollToBottom,
-    snapToBottom: () => {
-      const el = scroll
-      if (!el) return
+    resume: () => {
       if (store.userScrolled) setStore("userScrolled", false)
-      // With column-reverse, scrollTop=0 is at the bottom
-      el.scrollTop = 0
-      markProgrammatic()
+      scrollToBottom(true)
     },
+    scrollToBottom: () => scrollToBottom(false),
+    forceScrollToBottom: () => scrollToBottom(true),
     userScrolled: () => store.userScrolled,
   }
 }
diff --git a/packages/ui/src/hooks/index.ts b/packages/ui/src/hooks/index.ts
index 0fcf6f086..1c90a2e49 100644
--- a/packages/ui/src/hooks/index.ts
+++ b/packages/ui/src/hooks/index.ts
@@ -1,3 +1,2 @@
 export * from "./use-filtered-list"
 export * from "./create-auto-scroll"
-export * from "./use-reduced-motion"
diff --git a/packages/ui/src/hooks/use-reduced-motion.ts b/packages/ui/src/hooks/use-reduced-motion.ts
deleted file mode 100644
index 0038760ec..000000000
--- a/packages/ui/src/hooks/use-reduced-motion.ts
+++ /dev/null
@@ -1,10 +0,0 @@
-import { isHydrated } from "@solid-primitives/lifecycle"
-import { createMediaQuery } from "@solid-primitives/media"
-import { createHydratableSingletonRoot } from "@solid-primitives/rootless"
-
-const query = "(prefers-reduced-motion: reduce)"
-
-export const useReducedMotion = createHydratableSingletonRoot(() => {
-  const value = createMediaQuery(query)
-  return () => !isHydrated() || value()
-})