[GTK] Tree inset marks are cleared if feedback is updated during "drag over" event

[ptziegler]

Describe the bug

This issue was originally reported in GEF. During a drag & drop operation on a tree are explicitly set using setTreeInsets(...). However, those insets are immediately removed when updating the feedback field of the DropTargetEvent event.

To Reproduce

package org.eclipse.swt.tests.gtk.snippets;

import org.eclipse.swt.SWT;
import org.eclipse.swt.dnd.DND;
import org.eclipse.swt.dnd.DragSource;
import org.eclipse.swt.dnd.DragSourceAdapter;
import org.eclipse.swt.dnd.DragSourceEvent;
import org.eclipse.swt.dnd.DragSourceListener;
import org.eclipse.swt.dnd.DropTarget;
import org.eclipse.swt.dnd.DropTargetAdapter;
import org.eclipse.swt.dnd.DropTargetEvent;
import org.eclipse.swt.dnd.DropTargetListener;
import org.eclipse.swt.dnd.TextTransfer;
import org.eclipse.swt.graphics.Point;
import org.eclipse.swt.layout.FillLayout;
import org.eclipse.swt.widgets.Display;
import org.eclipse.swt.widgets.Shell;
import org.eclipse.swt.widgets.Tree;
import org.eclipse.swt.widgets.TreeItem;

public class Bug_TreeInsets {
	public static void main(String[] args) {
		Display display = new Display();
		Shell shell = new Shell(display);
		shell.setLayout(new FillLayout());

		Tree tree = new Tree(shell, SWT.NONE);
		for (int i = 1; i <= 10; ++i) {
			TreeItem treeItem = new TreeItem(tree, SWT.NONE);
			treeItem.setText("Tree Item " + i);
		}

		final DragSourceListener dragListener = new DragSourceAdapter() {
			@Override
			public void dragSetData(DragSourceEvent event) {
				TreeItem[] selection = tree.getSelection();
				if (selection.length > 0) {
					event.data = selection[0].getText();
				}
			}
		};
		final DragSource dragSource = new DragSource(tree, DND.DROP_MOVE);
		dragSource.setTransfer(TextTransfer.getInstance());
		dragSource.addDragListener(dragListener);

		final DropTargetListener dropListener = new DropTargetAdapter() {
			@Override
			public void dragOver(DropTargetEvent event) {
				Point location = tree.toControl(event.x, event.y);
				TreeItem treeItem = tree.getItem(location);
				if (treeItem != null) {
					tree.setInsertMark(treeItem, false);
				}
				event.feedback = DND.FEEDBACK_SCROLL | DND.FEEDBACK_EXPAND;
			}

			@Override
			public void drop(DropTargetEvent event) {
				tree.setInsertMark(null, false);
			}
		};
		final DropTarget target = new DropTarget(tree, DND.DROP_MOVE);
		target.setTransfer(TextTransfer.getInstance());
		target.addDropListener(dropListener);


		shell.open();
		while (!shell.isDisposed()) {
			if (!display.readAndDispatch())
				display.sleep();
		}
		display.dispose();
	}
}

Expected behavior

The insets should appear around the tree item the cursor is currently hovering over.

Screenshots

Here the expected behavior on Windows. On Linux, those insets don't appear. Or rather, they appear for a split-second before being removed again.

Recording.2026-06-03.210606.mp4

Environment:

1. Select the platform(s) on which the behavior is seen:

• • All OS
• • Windows
• • Linux
• • macOS

2. Additional OS info (e.g. OS version, Linux Desktop, etc)

• Kubuntu 25.10

3. JRE/JDK version

Version since

I became aware of this problem in the WindowBuilder project regarding a comment that was made back in 2011. So this problem exists since forever.

/*
 * Feature in Linux: during DND dragOver() operation the
 * DropTargetEvent.feedback resets all previous tree insert marks.
 */

Workaround (or) Additional context

The DND.FEEDBACK_INSERT_BEFORE or thee DND.FEEDBACK_INSERT_AFTER flag needs to be explicitly set.