[FIXED JENKINS-22934] Show poll thread count option when necessary.

* Show the option when the configured value is not the default
* Show the option even when projects are in other ItemGroups
* Only consider AbstractProjects

core/src/main/java/hudson/triggers/SCMTrigger.java

@@ -45,6 +45,8 @@
 import hudson.util.SequentialExecutionQueue;
 import org.apache.commons.io.FileUtils;
 import org.apache.commons.jelly.XMLOutput;
+import org.kohsuke.accmod.Restricted;
+import org.kohsuke.accmod.restrictions.NoExternalUse;
 import org.kohsuke.stapler.StaplerRequest;
 import org.kohsuke.stapler.DataBoundConstructor;
 
@@ -260,6 +262,15 @@ public void setPollingThreadCount(int n) {
             resizeThreadPool();
         }
 
+        @Restricted(NoExternalUse.class)
+        public boolean isPollingThreadCountOptionVisible() {
+            // unless you have a fair number of projects, this option is likely pointless.
+            // so let's hide this option for new users to avoid confusing them
+            // unless it was already changed
+            return Jenkins.getInstance().getAllItems(AbstractProject.class).size() > 10
+                    || getPollingThreadCount() != 0;
+        }
+
         /**
          * Update the {@link ExecutorService} instance.
          */

core/src/main/resources/hudson/triggers/SCMTrigger/global.jelly

@@ -24,11 +24,7 @@ THE SOFTWARE.
 
 <?jelly escape-by-default='true'?>
 <j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" xmlns:d="jelly:define" xmlns:l="/lib/layout" xmlns:t="/lib/hudson" xmlns:f="/lib/form">
-  <j:if test="${app.items.size()>10}">
-    <!--
-      unless you have a fair number of projects, this option is likely pointless.
-      so let's hide this option for new users to avoid confusing them.
-    -->
+  <j:if test="${descriptor.pollingThreadCountOptionVisible}">
     <f:section title="${%SCM Polling}">
       <f:entry title="${%Max # of concurrent polling}" field="pollingThreadCount">
         <f:number value="${descriptor.pollingThreadCount==0 ? '' : descriptor.pollingThreadCount}"