NextTab action: allow Cycling when reaching the end

Specifically for CTabFolders, this commit detects when the NextTab
action has reached the last item and will cycle to the first item
instead of doing nothing. It will load the item which is displayed in
that page.

bundles/org.eclipse.ui.workbench/Eclipse UI/org/eclipse/ui/internal/handlers/TraversePageHandler.java

@@ -16,6 +16,8 @@
 import java.lang.reflect.Method;
 import org.eclipse.core.commands.ExecutionEvent;
 import org.eclipse.swt.SWT;
+import org.eclipse.swt.custom.CTabFolder;
+import org.eclipse.swt.custom.CTabItem;
 import org.eclipse.swt.widgets.Control;
 import org.eclipse.swt.widgets.Display;
 import org.eclipse.swt.widgets.Shell;
@@ -38,9 +40,15 @@ public class TraversePageHandler extends WidgetMethodHandler {
 	public final Object execute(final ExecutionEvent event) {
 		Control focusControl = Display.getCurrent().getFocusControl();
 		if (focusControl != null) {
-			int traversal = "next".equals(methodName) ? SWT.TRAVERSE_PAGE_NEXT : SWT.TRAVERSE_PAGE_PREVIOUS; //$NON-NLS-1$
+			boolean forward = "next".equals(methodName); //$NON-NLS-1$
+			int traversal = getTraversalDirection(forward);
 			Control control = focusControl;
 			do {
+				if (control instanceof CTabFolder folder && isFinalItemInCTabFolder(folder, forward)) {
+					loopToSecondToFirstItemInCTabFolder(folder, forward);
+					traversal = getTraversalDirection(!forward); // we are in the second-to-last item in the given
+					// direction. Now, use the Traverse-event to move back by one
+				}
 				if (control.traverse(traversal))
 					return null;
 				if (control instanceof Shell)
@@ -52,6 +60,45 @@ public final Object execute(final ExecutionEvent event) {
 		return null;
 	}
 
+	private int getTraversalDirection(boolean direction) {
+		return direction ? SWT.TRAVERSE_PAGE_NEXT : SWT.TRAVERSE_PAGE_PREVIOUS;
+	}
+
+	/**
+	 * Sets the current selection to the second-to-last item in the given direction.
+	 *
+	 * @param folder
+	 * @param forward
+	 */
+	private void loopToSecondToFirstItemInCTabFolder(CTabFolder folder, boolean forward) {
+		if (forward) {
+			folder.showItem(folder.getItem(0));
+			folder.setSelection(1);
+		} else {
+			int itemCount = folder.getItemCount();
+			folder.setSelection(itemCount - 2);
+		}
+	}
+
+	/**
+	 * {@return Returns whether the folder has currently selected the final item in
+	 * the given direction.}
+	 *
+	 * @param folder  the CTabFolder which we want to inspect
+	 * @param forward whether we want to traverse forwards of backwards
+	 */
+	private boolean isFinalItemInCTabFolder(CTabFolder folder, boolean forward) {
+		CTabItem currentFolder = folder.getSelection();
+		CTabItem lastFolder = null;
+		if (forward) {
+			int itemCount = folder.getItemCount();
+			lastFolder = folder.getItem(itemCount - 1);
+		} else {
+			lastFolder = folder.getItem(0);
+		}
+		return currentFolder.equals(lastFolder);
+	}
+
 	/**
 	 * Looks up the traverse(int) method on the given focus control.
 	 *