fix(h5): showToast position:fixed

src/platforms/h5/components/app/popup/toast.vue

 <template>
   <transition name="uni-fade">
     <uni-toast
-      v-show="visible"
-      :class="{'uni-toast-fixed':mask}"
+      v-if="visible"
       :data-duration="duration">
       <div
         v-if="mask"
@@ -86,20 +85,17 @@ export default {
 }
 </script>
 <style>
-	uni-toast {
+	uni-toast {
+    position: fixed;
+		top: 0;
+		right: 0;
+		bottom: 0;
+		left: 0;
+		z-index: 999;
 		display: block;
 		box-sizing: border-box;
 	}
 
-	uni-toast .uni-toast-fixed {
-		position: fixed;
-		top: 0;
-		right: 0;
-		bottom: 0;
-		left: 0;
-		z-index: 999;
-	}
-
 	uni-toast .uni-sample-toast {
 		position: fixed;
 		z-index: 999;