Run against dev platform JARs and hover the mouse over the long label:

---%<---
import java.awt.EventQueue;
import javax.swing.JFrame;
import javax.swing.WindowConstants;
import org.openide.explorer.ExplorerManager;
import org.openide.explorer.view.BeanTreeView;
import org.openide.nodes.AbstractNode;
import org.openide.nodes.Children;
import org.openide.nodes.Node;
public class LongToolTips extends JFrame implements ExplorerManager.Provider {
    ExplorerManager mgr = new ExplorerManager();
    LongToolTips() {
        super("LongToolTips");
        Children.Array ch = new Children.Array();
        Node parent = new AbstractNode(ch);
        parent.setDisplayName("parent");
        Node n1 = new AbstractNode(Children.LEAF);
        n1.setDisplayName("short name");
        Node n2 = new AbstractNode(Children.LEAF);
        StringBuilder b = new StringBuilder("long name: ");
        for (int i = 1; i <= 100; i++) {
            b.append(" some part of the message #" + i + " ");
        }
        n2.setDisplayName(b.toString());
        ch.add(new Node[] {n1, n2});
        mgr.setRootContext(parent);
        add(new BeanTreeView());
        pack();
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        setVisible(true);
    }
    public ExplorerManager getExplorerManager() {
        return mgr;
    }
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                new LongToolTips();
            }
        });
    }
}
---%<---

On JDK 5/6, it displays fine. On JDK 7 (b04), the tool tip flickers gray rapidly
and then sticks: a grey window of one text line height appears across the entire
width of the screen and cannot easily be gotten rid of.

This is a significant problem when using the IDE on JDK 7. If you are
unfortunate enough to hover over a unit test failure message (in the Results
window) which is longer than the screen, you will be stuck with a grey bar on
top of your IDE, blocking text etc., until you restart.