Stop DOM nodes from claiming inherited members as own properties

src/AngleSharp.Js.Tests/DomTests.cs

@@ -82,5 +82,33 @@ public async Task ConsoleKeepsPropertiesAssignedToIt()
             var result = await "(function () { window.console.marker = 'kept'; return window.console.marker; })()".EvalScriptAsync();
             Assert.AreEqual("kept", result);
         }
+
+        [Test]
+        public async Task InheritedMemberIsNotAnOwnPropertyOfTheNode()
+        {
+            var result = await "document.createElement('div').hasOwnProperty('firstChild')".EvalScriptAsync();
+            Assert.AreEqual("False", result);
+        }
+
+        [Test]
+        public async Task InheritedMemberIsStillVisibleOnTheNode()
+        {
+            var result = await "('firstChild' in document.documentElement) + ',' + (typeof document.documentElement.appendChild)".EvalScriptAsync();
+            Assert.AreEqual("true,function", result);
+        }
+
+        [Test]
+        public async Task InheritedAccessorStillReadsAndWrites()
+        {
+            var result = await "(function () { var d = document.createElement('div'); d.id = 'jint'; return d.id; })()".EvalScriptAsync();
+            Assert.AreEqual("jint", result);
+        }
+
+        [Test]
+        public async Task AssignedPropertyIsReportedConsistently()
+        {
+            var result = await "(function () { var d = document.createElement('div'); d.custom = 1; return d.hasOwnProperty('custom') + ',' + Object.getOwnPropertyNames(d).join(); })()".EvalScriptAsync();
+            Assert.AreEqual("true,custom", result);
+        }
     }
 }

src/AngleSharp.Js/Proxies/DomNodeInstance.cs

@@ -67,18 +67,14 @@ public DomEventInstance.Registration RemoveEventHandler(DomEventInstance ev)
 
         public override PropertyDescriptor GetOwnProperty(JsValue property)
         {
-            if (Prototype is DomPrototypeInstance prototype)
+            //  An indexer is the only thing that can turn into an own property of the node
+            //  itself. The members of the DOM interface live on the prototype, so finding
+            //  them is the engine's job - answering them here would make the node claim
+            //  every inherited member as its own.
+            if (Prototype is DomPrototypeInstance prototype &&
+                prototype.TryGetFromIndex(_value, property.ToString(), out var descriptor))
             {
-                if (prototype.TryGetFromIndex(_value, property.ToString(), out var descriptor))
-                {
-                    return descriptor;
-                }
-
-                var prototypeProperty = prototype.GetOwnProperty(property);
-                if (prototypeProperty != PropertyDescriptor.Undefined)
-                {
-                    return prototypeProperty;
-                }
+                return descriptor;
             }
 
             return base.GetOwnProperty(property);