Fixed positioning with non-static parents

This fixes an issue with non-static parents (ones that were relatively
or absolutely positioned) where the dropdown would be offset by the
offset of the parent, so it would almost never line up. This fixes the
problem by removing the offset when the parent is not statically
positioned, which is what we used to do in older versions of Select2.

This closes https://github.com/select2/select2/issues/3303.
This closes https://github.com/select2/select2/issues/3614.
This closes https://github.com/select2/select2/issues/3672.

src/js/select2/dropdown/attachBody.js

@@ -3,7 +3,7 @@ define([
   '../utils'
 ], function ($, Utils) {
   function AttachBody (decorated, $element, options) {
-    this.$dropdownParent = options.get('dropdownParent') || document.body;
+    this.$dropdownParent = options.get('dropdownParent') || $(document.body);
 
     decorated.call(this, $element, options);
   }
@@ -157,6 +157,14 @@ define([
       top: container.bottom
     };
 
+    // Fix positioning with static parents
+    if (this.$dropdownParent[0].style.position !== 'static') {
+      var parentOffset = this.$dropdownParent.offset();
+
+      css.top -= parentOffset.top;
+      css.left -= parentOffset.left;
+    }
+
     if (!isCurrentlyAbove && !isCurrentlyBelow) {
       newDirection = 'below';
     }