fix(input): don't dirty model when input event triggered due to placeholder change

Certain versions of IE inexplicably trigger an input event in response to a placeholder
being set.

It is not possible to sniff for this behaviour nicely as the event is not triggered if
the element is not attached to the document, and the event triggers asynchronously so
it is not possible to accomplish this without deferring DOM compilation and slowing down
load times.

Closes #2614
Closes #5960

src/ng/directive/input.js

@@ -880,6 +880,7 @@ function addNativeHtml5Validators(ctrl, validatorName, element) {
 
 function textInputType(scope, element, attr, ctrl, $sniffer, $browser) {
   var validity = element.prop('validity');
+  var placeholder = element[0].placeholder, noevent = {};
 
   // In composition mode, users are still inputing intermediate text buffer,
   // hold the listener until composition is done.
@@ -902,6 +903,15 @@ function textInputType(scope, element, attr, ctrl, $sniffer, $browser) {
     var value = element.val(),
         event = ev && ev.type;
 
+    // IE (11 and under) seem to emit an 'input' event if the placeholder value changes. 
+    // We don't want to dirty the value when this happens, so we abort here. Unfortunately,
+    // IE also sends input events for other non-input-related things, (such as focusing on a
+    // form control), so this change is not entirely enough to solve this.
+    if (msie && (ev || noevent).type === 'input' && element[0].placeholder !== placeholder) {
+      placeholder = element[0].placeholder;
+      return;
+    }
+
     // By default we will trim the value
     // If the attribute ng-trim exists we will avoid trimming
     // e.g. <input ng-model="foo" ng-trim="false">

test/ng/directive/inputSpec.js

@@ -520,6 +520,23 @@ describe('input', function() {
     }
   });
 
+  it('should not dirty the model on an input event in response to a placeholder change', inject(function($sniffer) {
+    if (msie && $sniffer.hasEvent('input')) {
+      compileInput('<input type="text" ng-model="name" name="name" />');
+      inputElm.attr('placeholder', 'Test');
+      browserTrigger(inputElm, 'input');
+
+      expect(inputElm.attr('placeholder')).toBe('Test');
+      expect(inputElm).toBePristine();
+
+      inputElm.attr('placeholder', 'Test Again');
+      browserTrigger(inputElm, 'input');
+
+      expect(inputElm.attr('placeholder')).toBe('Test Again');
+      expect(inputElm).toBePristine();
+    }
+  }));
+
   describe('"change" event', function() {
     function assertBrowserSupportsChangeEvent(inputEventSupported) {
       // Force browser to report a lack of an 'input' event