[JENKINS-26781] lookup descriptor by ID, then by class if explicitly set

to match some specific use cases.

core/src/main/java/hudson/model/Descriptor.java

@@ -909,14 +909,19 @@ private URL getStaticHelpUrl(Klass<?> c, String suffix) {
         if (formData!=null) {
             for (Object o : JSONArray.fromObject(formData)) {
                 JSONObject jo = (JSONObject)o;
-                String kind = jo.optString("$class", null);
-                if (kind == null) {
-                  // Legacy: Remove once plugins have been staged onto $class
-                  kind = jo.getString("kind");
+                Descriptor<T> d = null;
+                String kind = jo.optString("kind", null);
+                if (kind != null) {
+                    d = find(descriptors, kind);
+                }
+                if (d == null) {
+                  kind = jo.getString("$class");
+                  d = find(descriptors, kind);
                 }
-                Descriptor<T> d = find(descriptors, kind);
                 if (d != null) {
                     items.add(d.newInstance(req, jo));
+                } else {
+                    LOGGER.warning("Received unexpected formData for descriptor " + kind);
                 }
             }
         }

core/src/main/resources/lib/form/class-entry.jelly

@@ -36,12 +36,11 @@ THE SOFTWARE.
   </st:documentation>
   <j:set var="clazz" value="${attrs.clazz ?: attrs.descriptor.clazz.name}" />
   <f:invisibleEntry>
-    <!-- Legacy: Remove once plugins have been staged onto $class -->
-    <input type="hidden" name="stapler-class" value="${clazz}" />
-    <!-- Legacy: Remove once plugins have been staged onto $class -->
     <j:if test="${attrs.descriptor != null}">
       <input type="hidden" name="kind" value="${attrs.descriptor.id}" />
     </j:if>
+    <!-- Legacy: Remove once plugins have been staged onto $class -->
+    <input type="hidden" name="stapler-class" value="${clazz}" />
     <input type="hidden" name="$class" value="${clazz}" />
   </f:invisibleEntry>
-</j:jelly>
+</j:jelly>