fix: correct home mask threshold after early scroll

src/pages/index/index.scss

@@ -261,7 +261,6 @@
   z-index: 40;
   pointer-events: none;
   transform: translateZ(0);
-  will-change: opacity;
 }
 
 .theme-light .home-top-mask {

src/pages/index/index.tsx

@@ -1,6 +1,6 @@
 import { View, Text, Image, Input, Button, Swiper, SwiperItem } from '@tarojs/components'
 import Taro from '@tarojs/taro'
-import { useEffect, useRef, useState } from 'react'
+import { useCallback, useEffect, useMemo, useRef, useState } from 'react'
 import './index.scss'
 import { CATEGORIES } from '../../utils/productConfig'
 import { assetUrl } from '../../utils/asset'
@@ -8,7 +8,7 @@ import { useThemeContext } from '../../context/ThemeContext'
 import { useSafeArea } from '../../hooks/useSafeArea'
 import { useStatusBar } from '../../hooks/useStatusBar'
 import ThemedPageMeta from '../../components/ThemedPageMeta'
-import { getTopMaskProgress } from '../../utils/homeTopMask'
+import { getTopMaskProgress, getTopMaskStartY } from '../../utils/homeTopMask'
 
 // 成品展示配图（从 img 里取对应实物图）
 const SHOWCASE_LIST = [
@@ -72,11 +72,15 @@ export default function Index() {
   const [searchKey, setSearchKey] = useState('')
 
   const [topMaskOpacity, setTopMaskOpacity] = useState(0)
-  const titleMetricsRef = useRef({ startY: 0, rangeY: 48 })
+  const fallbackTitleMetrics = useMemo(() => ({
+    startY: Math.max(0, safe.headerPaddingTop - safe.statusBarHeight),
+    rangeY: Math.max(48, safe.headerPaddingTop * 0.45)
+  }), [safe.headerPaddingTop, safe.statusBarHeight])
+  const titleMetricsRef = useRef(fallbackTitleMetrics)
   const scrollFrameRef = useRef<ReturnType<typeof setTimeout> | null>(null)
   const latestScrollTopRef = useRef(0)
 
-  const applyTopMaskProgress = (scrollTop: number) => {
+  const applyTopMaskProgress = useCallback((scrollTop: number) => {
     latestScrollTopRef.current = scrollTop
     if (scrollFrameRef.current !== null) {
       return
@@ -87,29 +91,33 @@ export default function Index() {
       const { startY, rangeY } = titleMetricsRef.current
       setTopMaskOpacity(getTopMaskProgress({ scrollTop: latestScrollTopRef.current, startY, rangeY }))
     }, 16)
-  }
+  }, [])
 
   useEffect(() => {
     let cancelled = false
     let measureTimer: ReturnType<typeof setTimeout> | null = null
 
+    titleMetricsRef.current = fallbackTitleMetrics
+
+    type HomeHeaderRect = Taro.NodesRef.BoundingClientRectCallbackResult
+
     const measureTitle = () => {
       if (cancelled) return
       Taro.createSelectorQuery()
         .select('.home-header')
-        .boundingClientRect((rect: any) => {
+        .boundingClientRect((rect: HomeHeaderRect | HomeHeaderRect[]) => {
           if (cancelled) return
-          if (!rect || typeof rect.bottom !== 'number' || typeof rect.height !== 'number') {
-            const fallbackRange = Math.max(48, safe.headerPaddingTop * 0.45)
-            titleMetricsRef.current = {
-              startY: Math.max(0, safe.headerPaddingTop - safe.statusBarHeight),
-              rangeY: fallbackRange
-            }
+          if (!rect || Array.isArray(rect) || typeof rect.bottom !== 'number' || typeof rect.height !== 'number') {
+            titleMetricsRef.current = fallbackTitleMetrics
             applyTopMaskProgress(latestScrollTopRef.current)
             return
           }
 
-          const startY = Math.max(0, rect.bottom - safe.statusBarHeight - 8)
+          const startY = getTopMaskStartY({
+            viewportBottom: rect.bottom,
+            scrollTop: latestScrollTopRef.current,
+            statusBarHeight: safe.statusBarHeight
+          })
           const rangeY = Math.max(32, rect.height * 0.7)
           titleMetricsRef.current = { startY, rangeY }
           applyTopMaskProgress(latestScrollTopRef.current)
@@ -132,7 +140,7 @@ export default function Index() {
       }
       scrollFrameRef.current = null
     }
-  }, [safe.headerPaddingTop, safe.statusBarHeight])
+  }, [applyTopMaskProgress, fallbackTitleMetrics])
 
   Taro.usePageScroll((e) => {
     applyTopMaskProgress(e.scrollTop)

src/utils/homeTopMask.js

@@ -1,3 +1,16 @@
+/**
+ * Convert a viewport-relative header bottom into a document-relative mask
+ * threshold. Selector queries report coordinates relative to the current
+ * viewport, so the scroll offset at measurement time must be added back.
+ */
+export function getTopMaskStartY({ viewportBottom, scrollTop, statusBarHeight }) {
+  const safeViewportBottom = Number.isFinite(viewportBottom) ? viewportBottom : 0
+  const safeScrollTop = Number.isFinite(scrollTop) ? scrollTop : 0
+  const safeStatusBarHeight = Number.isFinite(statusBarHeight) ? statusBarHeight : 0
+
+  return Math.max(0, safeViewportBottom + safeScrollTop - safeStatusBarHeight - 8)
+}
+
 export function getTopMaskProgress({ scrollTop, startY, rangeY }) {
   const safeScrollTop = Number.isFinite(scrollTop) ? scrollTop : 0
   const start = Math.max(0, Number.isFinite(startY) ? startY : 0)