ADDITIONAL SYSTEM INFORMATION :
Currently every system / Java is affected

A DESCRIPTION OF THE PROBLEM :
A developer should be able to force the tooltips to be displayed in a heavyweight popup rather than in a medium weight popup.

At the moment you can use illegal reflective access to force the PopupManager to display the popup in a heavyweight popup with the following code:

protected void installDefaults(JPanel p) {
    super.installDefaults(p);
    p.setFont(AbstractLookAndFeel.getTheme().getControlTextFont());

    // We don't want medium weight popups for tool tips, so we try to force heavy weight popups.
    try {
        Class clazz = Class.forName("javax.swing.ClientPropertyKey");
        Field field = clazz.getDeclaredField("PopupFactory_FORCE_HEAVYWEIGHT_POPUP");
        field.setAccessible(true);
        p.putClientProperty(field.get(null), Boolean.TRUE);
    } catch (Exception ex) {
    }
}

But since such a reflective access to the enum javax.swing.ClientPropertyKey is illegal (and will be removed in future releases) we do not have any chance to force the tooltips to be shown in a heavyweight popup.

In the JPopupMenu you have the option to set "setLightWeightPopupEnabled(false)" to force a heavywight popup because of

if (isLightWeightPopupEnabled()) {
    popupFactory.setPopupType(PopupFactory.LIGHT_WEIGHT_POPUP);
} else {
    popupFactory.setPopupType(PopupFactory.HEAVY_WEIGHT_POPUP);
}

but if you do the same in the "ToolTipManager.setLightWeightPopupEnabled(false)" you can only force a medium weight popup because of

if (lightWeightPopupEnabled) {
    int y = getPopupFitHeight(popupRect, insideComponent);
    int x = getPopupFitWidth(popupRect,insideComponent);
    if (x>0 || y>0) {
        popupFactory.setPopupType(PopupFactory.MEDIUM_WEIGHT_POPUP);
    } else {
        popupFactory.setPopupType(PopupFactory.LIGHT_WEIGHT_POPUP);
    }
}
else {
    popupFactory.setPopupType(PopupFactory.MEDIUM_WEIGHT_POPUP);
}

In addition, there is also a field "heavyWeightPopupEnabled" inside the ToolTipManager which is not used..
There should be at least one way to force a heavy weight popup in the ToolTipManager. Furthermore a different behavior between the JPopupMenu.setLightWeightPopupEnabled(false) and ToolTipManager.setLightWeightPopupEnabled(false) should be avoided!

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Just add a JLabel to a JPanel and set a tooltip to the JLabel. Check if the tooltip is being rendered in a lightweight or mediumweight popup.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
you should be able to force the tooltip to be rendered in a heavyweight popup either via

ToolTipManager.setLightWeightPopupEnabled(false) (to be consistent with the JPopupMenu) or add a new method ToolTipManager.setHeavyWeightPopupEnabled(false) (to use the unused field heavyWeightPopupEnabled inside the ToolTipManager)
ACTUAL -
you need reflection to achieve the tooltip to be displayed in a heavyweight popup which will not be possible any more in future releases

---------- BEGIN SOURCE ----------
// -*- mode:java; encoding:utf-8 -*-
// vim:set fileencoding=utf-8:
// @homepage@

package example;

import java.awt.*;
import java.lang.reflect.Field;
import java.security.AccessController;
import java.security.PrivilegedAction;
import javax.swing.*;

public final class MainPanel extends JPanel {
  private MainPanel() {
    super();
    JLabel label = makeLabel("FORCE_HEAVYWEIGHT_POPUP", Color.PINK);

    ToolTipManager.sharedInstance().setLightWeightPopupEnabled(false);
    AccessController.doPrivileged(new PrivilegedAction<Void>() {
      @Override public Void run() {
        try {
          Field field;
          if (System.getProperty("java.version").startsWith("1.6.0")) {
            // https://community.oracle.com/thread/1357949 ComboBox scroll and selected/highlight on glasspane
            // Class<?> clazz = Class.forName("javax.swing.PopupFactory"); // errorprone: LiteralClassName
            // field = clazz.getDeclaredField("forceHeavyWeightPopupKey");
            field = PopupFactory.class.getDeclaredField("forceHeavyWeightPopupKey");
          } else { // JDK 1.7.0, 1.8.0
            Class<?> clazz = Class.forName("javax.swing.ClientPropertyKey");
            field = clazz.getDeclaredField("PopupFactory_FORCE_HEAVYWEIGHT_POPUP");
          }
          field.setAccessible(true);
          label.putClientProperty(field.get(null), Boolean.TRUE);
        } catch (ClassNotFoundException | NoSuchFieldException | IllegalAccessException ex) {
          throw new UnsupportedOperationException(ex);
        }
        return null;
      }
    });

    JComponent glass = new JPanel(new BorderLayout()) {
      private final Color backgroundColor = new Color(0x64_64_64_C8, true);
      @Override protected void paintComponent(Graphics g) {
        g.setColor(backgroundColor);
        g.fillRect(0, 0, getWidth(), getHeight());
        super.paintComponent(g);
      }
    };
    glass.setOpaque(false);
    glass.add(makeLabel("Default: ToolTipText", Color.ORANGE), BorderLayout.WEST);
    glass.add(label, BorderLayout.EAST);
    glass.add(Box.createVerticalStrut(60), BorderLayout.SOUTH);
    EventQueue.invokeLater(() -> {
      getRootPane().setGlassPane(glass);
      getRootPane().getGlassPane().setVisible(true);
    });
    setPreferredSize(new Dimension(320, 240));
  }

  private static JLabel makeLabel(String title, Color color) {
    JLabel label = new JLabel(title);
    label.setOpaque(true);
    label.setBackground(color);
    label.setToolTipText("1234567890");
    return label;
  }

  public static void main(String[] args) {
    EventQueue.invokeLater(MainPanel::createAndShowGui);
  }

  private static void createAndShowGui() {
    try {
      UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
    } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
      ex.printStackTrace();
      Toolkit.getDefaultToolkit().beep();
    }
    JFrame frame = new JFrame("@title@");
    frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    frame.getContentPane().add(new MainPanel());
    frame.pack();
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
  }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
actual workaround is reflection, but this will not work in future releases

FREQUENCY : always