Perform element changes always in a non UI thread

Currently PackageExplorerContentProvider receives events for changed
elements from different threads and already has handling to perform only
certain actions in the UI thread. But it does not really account for the
case when the event itself is triggered from the UI thread in which case
lengthy operation can take place in the UI.

This now checks if the change is using the UI and otherwise schedules a
job to perform the required actions in the background.

org.eclipse.jdt.ui.tests/ui/org/eclipse/jdt/ui/tests/packageview/ContentProviderTests3.java

@@ -30,6 +30,7 @@
 import org.eclipse.jdt.testplugin.JavaTestPlugin;
 
 import org.eclipse.core.runtime.Path;
+import org.eclipse.core.runtime.jobs.Job;
 
 import org.eclipse.core.resources.IFile;
 import org.eclipse.core.resources.IFolder;
@@ -202,9 +203,14 @@ public void testDeleteBottomLevelFragmentFolding() throws Exception {
 		//send a delta indicating fragment deleted
 		IElementChangedListener listener= (IElementChangedListener) fProvider;
 		IJavaElementDelta delta= TestDelta.createDelta(fPack4, IJavaElementDelta.REMOVED);
-		listener.elementChanged(new ElementChangedEvent(delta, ElementChangedEvent.POST_CHANGE));
-
-		//force events from dispaly
+		//using a job prevents running the update in the UI what would internally create a job so better use one we can control...
+		Job job= Job.create("ElementChangeTestJob", m->{
+			listener.elementChanged(new ElementChangedEvent(delta, ElementChangedEvent.POST_CHANGE));
+		});
+		job.schedule();
+		//wait for job
+		job.join();
+		//force events from display
 		while(fMyPart.getTreeViewer().getControl().getDisplay().readAndDispatch())
 
 		assertTrue(fMyPart.hasRefreshHappened(), "Refresh happened"); //$NON-NLS-1$

org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/packageview/PackageExplorerContentProvider.java

@@ -28,6 +28,7 @@
 import org.eclipse.core.runtime.IProgressMonitor;
 import org.eclipse.core.runtime.IStatus;
 import org.eclipse.core.runtime.Status;
+import org.eclipse.core.runtime.jobs.Job;
 
 import org.eclipse.core.resources.IFolder;
 import org.eclipse.core.resources.IProject;
@@ -127,6 +128,20 @@ protected Object getViewerInput() {
 
 	@Override
 	public void elementChanged(final ElementChangedEvent event) {
+		IJavaElementDelta delta= event.getDelta();
+		if (Display.findDisplay(Thread.currentThread()) == null) {
+			processDelta(delta);
+		} else {
+			Job job= Job.create(PackagesMessages.PackageExplorerContentProvider_update_job_description, m -> {
+				processDelta(delta);
+				return m.isCanceled() ? Status.CANCEL_STATUS : Status.OK_STATUS;
+			});
+			job.setSystem(true);
+			job.schedule();
+		}
+	}
+
+	protected void processDelta(IJavaElementDelta delta) {
 		final ArrayList<Runnable> runnables= new ArrayList<>();
 		try {
 			clearPackageCache();
@@ -136,7 +151,7 @@ public void elementChanged(final ElementChangedEvent event) {
 			if (inputDeleted(runnables))
 				return;
 
-			processDelta(event.getDelta(), runnables);
+			processDelta(delta, runnables);
 		} catch (JavaModelException e) {
 			JavaPlugin.log(e);
 		} finally {