Modal: Work around IE scrollbars not taking up page width

Fixes #15353.

js/modal.js

@@ -248,7 +248,12 @@
   }
 
   Modal.prototype.checkScrollbar = function () {
-    this.bodyIsOverflowing = document.body.scrollHeight > document.documentElement.clientHeight
+    var fullWindowWidth = window.innerWidth
+    if (!fullWindowWidth) { // workaround for missing window.innerWidth in IE8
+      var documentElementRect = document.documentElement.getBoundingClientRect()
+      fullWindowWidth = documentElementRect.right - Math.abs(documentElementRect.left)
+    }
+    this.bodyIsOverflowing = document.body.clientWidth < fullWindowWidth
     this.scrollbarWidth = this.measureScrollbar()
   }