fix: app-v3 原生组件支持 fixed 定位

src/platforms/app-plus/view/mixins/native.js

+const TITLEBAR_HEIGHT = 44
+
+function getStatusbarHeight () {
+  // 横屏时 iOS 获取的状态栏高度错误，进行纠正
+  return plus.navigator.isImmersedStatusbar() ? Math.round(plus.os.name === 'iOS' ? plus.navigator.getSafeAreaInsets().top : plus.navigator.getStatusbarHeight()) : 0
+}
+
+function getNavigationBarHeight () {
+  const webview = plus.webview.currentWebview()
+  let style = webview.getStyle()
+  style = style && style.titleNView
+  if (style && style.type === 'default') {
+    return TITLEBAR_HEIGHT + getStatusbarHeight()
+  }
+  return 0
+}
+
+function getFixed ($el) {
+  let fixed
+  while ($el) {
+    const style = getComputedStyle($el)
+    const transform = style.transform || style.webkitTransform
+    fixed = transform && transform !== 'none' ? false : fixed
+    fixed = style.position === 'fixed' ? true : fixed
+    $el = $el.parentElement
+  }
+  return fixed
+}
+
 export default {
   name: 'Native',
   data () {
@@ -28,10 +57,13 @@ export default {
       const rect = (this.$refs.container || this.$el).getBoundingClientRect()
       this.hidden = rect.width === 0 || rect.height === 0
       if (!this.hidden) {
-        ['top', 'left', 'width', 'height'].forEach(key => {
+        const position = this.position
+        position.position = getFixed(this.$el) ? 'absolute' : 'static'
+        const keys = ['top', 'left', 'width', 'height']
+        keys.forEach(key => {
           let val = rect[key]
-          val = key === 'top' ? val + (document.documentElement.scrollTop || document.body.scrollTop || 0) : val
-          this.position[key] = val + 'px'
+          val = key === 'top' ? val + (position.position === 'static' ? (document.documentElement.scrollTop || document.body.scrollTop || 0) : getNavigationBarHeight()) : val
+          position[key] = val + 'px'
         })
       }
     },