Allow value to merge from previous behavior property declaration. Fixes

#5503

lib/legacy/class.js

@@ -155,6 +155,27 @@ function flattenBehaviors(behaviors, list, exclude) {
   return list;
 }
 
+/**
+ * Copies property descriptors from source to target, overwriting all fields
+ * of any previous descriptor for a property *except* for `value`, which is
+ * merged in from the target if it does not exist on the source.
+ * 
+ * @param {*} target Target properties object
+ * @param {*} source Source properties object
+ */
+function mergeProperties(target, source) {
+  for (const p in source) {
+    const targetInfo = target[p];
+    const value = targetInfo && targetInfo.value;
+    const sourceInfo = source[p];
+    if (sourceInfo.value === undefined && value !== undefined) {
+      target[p] = Object.assign({value}, sourceInfo);
+    } else {
+      target[p] = sourceInfo;
+    }
+  }
+}
+
 /* Note about construction and extension of legacy classes.
   [Changed in Q4 2018 to optimize performance.]
 
@@ -227,10 +248,10 @@ function GenerateClassFromInfo(info, Base, behaviors) {
       const properties = {};
       if (behaviorList) {
         for (let i=0; i < behaviorList.length; i++) {
-          Object.assign(properties, behaviorList[i].properties);
+          mergeProperties(properties, behaviorList[i].properties);
         }
       }
-      Object.assign(properties, info.properties);
+      mergeProperties(properties, info.properties);
       return properties;
     }
 

test/unit/behaviors.html

@@ -402,15 +402,15 @@
       behaviors: [
         {
           properties: {
-            foo: { value: true },
-            bar: { value: true}
+            foo: { value: 'a' },
+            bar: { value: 'a'}
           }
         },
         {
           properties: {
-            foo: { value: true },
+            foo: { value: 'b' },
             bar: String,
-            zot: {value: true}
+            zot: {value: 'b'}
           }
         },
 
@@ -587,9 +587,9 @@
 
   test('behavior default values can be overridden', function() {
     const el = fixture('override-default-value');
-    assert.notOk(el.foo);
-    assert.notOk(el.bar);
-    assert.notOk(el.zot);
+    assert.equal(el.foo, 'b');
+    assert.equal(el.bar, 'a');
+    assert.equal(el.zot, 'b');
   });
 
   test('readOnly not applied when property was previously observed', function() {