when Descriptor is manually added/removed, ExtensionList<Descriptor> needs to be updated.

core/src/main/java/hudson/DescriptorExtensionList.java

@@ -133,6 +133,19 @@ public class DescriptorExtensionList<T extends Describable<T>, D extends Descrip
         return null;
     }
 
+    @Override
+    public boolean add(D d) {
+        boolean r = super.add(d);
+        hudson.getExtensionList(Descriptor.class).add(d);
+        return r;
+    }
+
+    @Override
+    public boolean remove(Object o) {
+        hudson.getExtensionList(Descriptor.class).remove(o);
+        return super.remove(o);
+    }
+
     /**
      * {@link #load()} in the descriptor is not a real load activity, so locking against "this" is enough.
      */

core/src/main/java/hudson/ExtensionList.java

@@ -156,7 +156,7 @@ public class ExtensionList<T> extends AbstractList<T> {
     }
 
     @Override
-    public synchronized T remove(int index) {
+    public final synchronized T remove(int index) {
         T t = get(index);
         remove(t);
         return t;