Fix traversal of CTabFolders with PgUp/PgDown #2864

When traversing from an editor with tabs (e.g. a POM or MANIFEST
editor), which is a CTabFolder itself, I made sure that the traversal
happens from the top-level CTabFolder i.e. from the one that contains
all open editors instead of trying to traverse inside the editor itself.

Fixes #2864

Co-authored-by: Christopher Hermann <christopher.hermann@sap.com>
Co-authored-by: Matthias Becker <ma.becker@sap.com>

Author: 3 people

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

@@ -41,11 +41,12 @@ public final Object execute(final ExecutionEvent event) {
 			int traversalDirection = translateToTraversalDirection(forward);
 			Control control = focusControl;
 			do {
-				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 instanceof CTabFolder) {
+					CTabFolder folder = getTopLevelCTabFolderInParentHierarchy(control);
+					if (isFinalItemInCTabFolder(folder, forward) && !hasHiddenItem(folder)) {
+						loopToFirstOrLastItem(folder, forward);
+						traversalDirection = translateToTraversalDirection(!forward);
+					}
 				}
 				if (control.traverse(traversalDirection)) {
 					return null;
@@ -59,6 +60,22 @@ public final Object execute(final ExecutionEvent event) {
 		return null;
 	}
 
+	/**
+	 * @param c a {@code Control}.
+	 * @return the top-level {@code CTabFolder} in the parent hierarchy.
+	 */
+	private CTabFolder getTopLevelCTabFolderInParentHierarchy(Control c) {
+		Control current = c;
+		CTabFolder ret = null;
+		do {
+			if (current instanceof CTabFolder folder) {
+				ret = folder;
+			}
+			current = current.getParent();
+		} while (current != null);
+		return ret;
+	}
+
 	private boolean hasHiddenItem(CTabFolder folder) {
 		return Arrays.stream(folder.getItems()).anyMatch(i -> !i.isShowing());
 	}