fix: mask layer css not working (#621, #574)

Author: sheepzh

src/content-script/limit/modal/style/modal.css

@@ -13,7 +13,7 @@ body {
     width: 100vw;
     padding: 0;
     margin: 0;
-    position: absolute;
+    position: fixed;
     top: 0;
     left: 0;
     z-index: 999999;