Fix issue where affixed element has incorrect positioning

when used with a sticky footer layout.
Adds visual test for affix with sticky footer.

Closes #15694.
Refs #15032.

js/affix.js

@@ -78,7 +78,7 @@
     var offset       = this.options.offset
     var offsetTop    = offset.top
     var offsetBottom = offset.bottom
-    var scrollHeight = $(document.body).height()
+    var scrollHeight = Math.max($(document).height(), $(document.body).height())
 
     if (typeof offset != 'object')         offsetBottom = offsetTop = offset
     if (typeof offsetTop == 'function')    offsetTop    = offset.top(this.$element)