fix(ngStyle): skip setting empty value when new style has the property

Previously, all the properties in oldStyles are set to empty value once.
Using AngularJS with jQuery 3.3.1, this disables the CSS transition as
reported in jquery/jquery#4185.

Closes #16709

Author: itchyny

src/ng/directive/ngStyle.js

@@ -54,7 +54,14 @@
 var ngStyleDirective = ngDirective(function(scope, element, attr) {
   scope.$watchCollection(attr.ngStyle, function ngStyleWatchAction(newStyles, oldStyles) {
     if (oldStyles && (newStyles !== oldStyles)) {
-      forEach(oldStyles, function(val, style) { element.css(style, '');});
+      if (!newStyles) {
+        newStyles = {};
+      }
+      forEach(oldStyles, function(val, style) {
+        if (newStyles[style] == null) {
+          newStyles[style] = '';
+        }
+      });
     }
     if (newStyles) element.css(newStyles);
   });

test/ng/directive/ngStyleSpec.js

@@ -120,5 +120,38 @@ describe('ngStyle', function() {
       expect(element.css(preCompStyle)).not.toBe('88px');
       expect(element.css(postCompStyle)).not.toBe('99px');
     });
+
+    it('should clear style when the new model is null', function() {
+      scope.styleObj = {'height': '99px', 'width': '88px'};
+      scope.$apply();
+      expect(element.css(preCompStyle)).toBe('88px');
+      expect(element.css(postCompStyle)).toBe('99px');
+      scope.styleObj = null;
+      scope.$apply();
+      expect(element.css(preCompStyle)).not.toBe('88px');
+      expect(element.css(postCompStyle)).not.toBe('99px');
+    });
+
+    it('should clear style when the value is undefined or null', function() {
+      scope.styleObj = {'height': '99px', 'width': '88px'};
+      scope.$apply();
+      expect(element.css(preCompStyle)).toBe('88px');
+      expect(element.css(postCompStyle)).toBe('99px');
+      scope.styleObj = {'height': undefined, 'width': null};
+      scope.$apply();
+      expect(element.css(preCompStyle)).not.toBe('88px');
+      expect(element.css(postCompStyle)).not.toBe('99px');
+    });
+
+    it('should set style when the value is zero', function() {
+      scope.styleObj = {'height': '99px', 'width': '88px'};
+      scope.$apply();
+      expect(element.css(preCompStyle)).toBe('88px');
+      expect(element.css(postCompStyle)).toBe('99px');
+      scope.styleObj = {'height': 0, 'width': 0};
+      scope.$apply();
+      expect(element.css(preCompStyle)).toBe('0px');
+      expect(element.css(postCompStyle)).toBe('0px');
+    });
   });
 });