fix(textarea): take scroll distance into account when setting the height

The textarea resize handle wasn't taking into account the scroll distance, causing it to be off if the page was scrolled.

Fixes #8632.

Closes #8643

Author: crisbeto

src/components/input/input.js

@@ -366,18 +366,13 @@ function inputTextareaDirective($mdUtil, $window, $mdAria, $timeout, $mdGesture)
         });
     }
 
-    //ngModelCtrl.$setTouched();
-    //if( ngModelCtrl.$invalid ) containerCtrl.setInvalid();
-
     scope.$on('$destroy', function() {
       containerCtrl.setFocused(false);
       containerCtrl.setHasValue(false);
       containerCtrl.input = null;
     });
 
-    /**
-     *
-     */
+    /** Gets run through ngModel's pipeline and set the `has-value` class on the container. */
     function ngModelPipelineCheckValue(arg) {
       containerCtrl.setHasValue(!ngModelCtrl.$isEmpty(arg));
       return arg;
@@ -556,7 +551,7 @@ function inputTextareaDirective($mdUtil, $window, $mdAria, $timeout, $mdGesture)
 
         function onDrag(ev) {
           if (!isDragging) return;
-          element.css('height', startHeight + (ev.pointer.y - dragStart) + 'px');
+          element.css('height', startHeight + (ev.pointer.y - dragStart) - $mdUtil.scrollTop() + 'px');
         }
 
         function onDragEnd(ev) {