fix(select): Position incorrect if selection scrolled.

If the currently selected object scrolled into view because it
was further down in the list, the position of the menu was
incorrect upon opening because the `scrollTop` of the `contentNode`
was always `0` due to the ARIA changes which set it to
`display: none` upon closing.

Fix by modifying the display to `block` when we run the `isScrollable`
check and back afterward (usually `display: none` but we account for
other options).

Also fixes a slight `2px` positioning issue which made the menu not
perfectly line up with the underlying selection.

Fixes #6190.

src/components/select/select.js

@@ -1276,7 +1276,7 @@ function SelectProvider($$interimElementProvider) {
      * Calculate the
      */
     function calculateMenuPositions(scope, element, opts) {
-      var 
+      var
         containerNode = element[0],
         targetNode = opts.target[0].children[0], // target the label
         parentNode = $document[0].body,
@@ -1298,11 +1298,12 @@ function SelectProvider($$interimElementProvider) {
           bottom: bounds.bottom - (targetRect.top + targetRect.height)
         },
         maxWidth = parentRect.width - SELECT_EDGE_MARGIN * 2,
-        isScrollable = contentNode.scrollHeight > contentNode.offsetHeight,
         selectedNode = selectNode.querySelector('md-option[selected]'),
         optionNodes = selectNode.getElementsByTagName('md-option'),
         optgroupNodes = selectNode.getElementsByTagName('md-optgroup');
 
+      var isScrollable = calculateScrollable(element, contentNode);
+
       var loading = isPromiseLike(opts.loadingAsync);
       var centeredNode;
       if (!loading) {
@@ -1386,7 +1387,7 @@ function SelectProvider($$interimElementProvider) {
       } else {
         left = (targetRect.left + centeredRect.left - centeredRect.paddingLeft) + 2;
         top = Math.floor(targetRect.top + targetRect.height / 2 - centeredRect.height / 2 -
-            centeredRect.top + contentNode.scrollTop) + 4;
+            centeredRect.top + contentNode.scrollTop) + 2;
 
         transformOrigin = (centeredRect.left + targetRect.width / 2) + 'px ' +
           (centeredRect.top + centeredRect.height / 2 - contentNode.scrollTop) + 'px 0px';
@@ -1437,5 +1438,24 @@ function SelectProvider($$interimElementProvider) {
       height: node.offsetHeight
     } : {left: 0, top: 0, width: 0, height: 0};
   }
+
+  function calculateScrollable(element, contentNode) {
+    var isScrollable = false;
+
+    try {
+      var oldDisplay = element[0].style.display;
+
+      // Set the element's display to block so that this calculation is correct
+      element[0].style.display = 'block';
+
+      isScrollable = contentNode.scrollHeight > contentNode.offsetHeight;
+
+      // Reset it back afterwards
+      element[0].style.display = oldDisplay;
+    } finally {
+      // Nothing to do
+    }
+    return isScrollable;
+  }
 }