Add circular navigation between open tabs

When navigating the open tabs with Ctrl+Pg Up/Down and only if there is
no chevron (i.e. more tabs opened than one can show) then the tab next
to the last one is the first one and the tab previous to the first one
is the last one.

Author: jannisCode

bundles/org.eclipse.ui.workbench/eclipseui/org/eclipse/ui/internal/handlers/TraversePageHandler.java

@@ -12,14 +12,15 @@
  *     IBM Corporation - initial API and implementation
  *******************************************************************************/
 package org.eclipse.ui.internal.handlers;
-
 import java.lang.reflect.Method;
+import java.util.Arrays;
 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;
-
 /**
  * This handler is an adaptation of the widget method handler that implements
  * page traversal via {@link SWT#TRAVERSE_PAGE_NEXT} and
@@ -28,30 +29,77 @@
  * @since 3.5
  */
 public class TraversePageHandler extends WidgetMethodHandler {
-
 	/**
 	 * The parameters for traverse(int).
 	 */
 	private static final Class<?>[] METHOD_PARAMETERS = { int.class };
-
 	@Override
 	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 traversalDirection = translateToTraversalDirection(forward);
 			Control control = focusControl;
 			do {
-				if (control.traverse(traversal))
+				if (control instanceof CTabFolder folder && isFinalItemInCTabFolder(folder, forward)
+						&& !hasHiddenItem(folder)) {
+					loopToFirstOrLastItem(folder, forward);
+					traversalDirection = translateToTraversalDirection(!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(traversalDirection))
 					return null;
 				if (control instanceof Shell)
 					return null;
 				control = control.getParent();
 			} while (control != null);
 		}
-
 		return null;
 	}
 
+	private boolean hasHiddenItem(CTabFolder folder) {
+		return Arrays.stream(folder.getItems()).anyMatch(i -> !i.isShowing());
+	}
+
+	private int translateToTraversalDirection(boolean forward) {
+		return forward ? SWT.TRAVERSE_PAGE_NEXT : SWT.TRAVERSE_PAGE_PREVIOUS;
+	}
+	
+	/**
+	 * Sets the current selection to the first or last item the given direction.
+	 *
+	 * @param folder  the CTabFolder which we want to inspect
+	 * @param forward whether we want to traverse forwards of backwards
+	 */
+	private void loopToFirstOrLastItem(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.
 	 *
@@ -70,5 +118,4 @@ protected Method getMethodToExecute() {
 		}
 		return null;
 	}
-
-}
+}