fix(backdrop): re-introduce resize handler, general cleanup

* Re-introduces the resize handler that was accidentally removed in 4803b49. This fixes issues where the backdrop wouldn't adjust when the body resizes.
* Cleans up a few redundancies.

Closes #9249

src/components/backdrop/backdrop.js

@@ -20,7 +20,7 @@
 angular
   .module('material.components.backdrop', ['material.core'])
   .directive('mdBackdrop', function BackdropDirective($mdTheming, $mdUtil, $animate, $rootElement, $window, $log, $$rAF, $document) {
-    var ERROR_CSS_POSITION = "<md-backdrop> may not work properly in a scrolled, static-positioned parent container.";
+    var ERROR_CSS_POSITION = '<md-backdrop> may not work properly in a scrolled, static-positioned parent container.';
 
     return {
       restrict: 'E',
@@ -31,48 +31,53 @@ angular
       // backdrop may be outside the $rootElement, tell ngAnimate to animate regardless
       if ($animate.pin) $animate.pin(element, $rootElement);
 
-      var bodyRect;
-      $$rAF(function () {
+      var bodyStyles;
+
+      $$rAF(function() {
         // If body scrolling has been disabled using mdUtil.disableBodyScroll(),
         // adjust the 'backdrop' height to account for the fixed 'body' top offset.
         // Note that this can be pretty expensive and is better done inside the $$rAF.
-        bodyRect = $window.getComputedStyle($document[0].body);
-        if (bodyRect.position == 'fixed') {
-          var hViewport = parseInt(bodyRect.height, 10) + Math.abs(parseInt(bodyRect.top, 10));
-          element.css({
-            height: hViewport + 'px'
+        bodyStyles = $window.getComputedStyle($document[0].body);
+
+        if (bodyStyles.position === 'fixed') {
+          var resizeHandler = $mdUtil.debounce(function(){
+            bodyStyles = $window.getComputedStyle($document[0].body);
+            resize();
+          }, 60, null, false);
+
+          resize();
+          angular.element($window).on('resize', resizeHandler);
+
+          scope.$on('$destroy', function() {
+            angular.element($window).off('resize', resizeHandler);
           });
         }
 
         // Often $animate.enter() is used to append the backDrop element
         // so let's wait until $animate is done...
-        var parent = element.parent()[0];
-        if (parent) {
+        var parent = element.parent();
 
-          if ( parent.nodeName == 'BODY' ) {
-            element.css({position : 'fixed'});
+        if (parent.length) {
+          if (parent[0].nodeName === 'BODY') {
+            element.css('position', 'fixed');
           }
 
-          var styles = $window.getComputedStyle(parent);
-          if (styles.position == 'static') {
+          var styles = $window.getComputedStyle(parent[0]);
+
+          if (styles.position === 'static') {
             // backdrop uses position:absolute and will not work properly with parent position:static (default)
             $log.warn(ERROR_CSS_POSITION);
           }
-        }
 
-        // Only inherit the parent if the backdrop has a parent.
-        if (element.parent().length) {
-          $mdTheming.inherit(element, element.parent());
+          // Only inherit the parent if the backdrop has a parent.
+          $mdTheming.inherit(element, parent);
         }
       });
 
       function resize() {
-        var hViewport = parseInt(bodyRect.height, 10) + Math.abs(parseInt(bodyRect.top, 10));
-        element.css({
-          height: hViewport + 'px'
-        });
+        var viewportHeight = parseInt(bodyStyles.height, 10) + Math.abs(parseInt(bodyStyles.top, 10));
+        element.css('height', viewportHeight + 'px');
       }
-
     }
 
   });