Why JScrollPane does not react to mouse wheel events?

Walter beat me to analysing the issue 🙂

Adding a bit of detail:

It’s correct that a JScrollPane supports mouseWheelHandling. According to the rules of mouseEvent dispatching, the top-most (in z-order) component gets the event, and that’s the scrollPane around the textArea. So if wheeling the textarea is not required, a simple solution might be to disable the wheel-support in its scrollPane. And JScrollPane even has api for doing it:

scrollPane.setWheelScrollingEnabled(false); 

Unfortunately, that doesn’t work. Reason it’s not working is that this property has no effect in the event dispatch chain which ultimately calls into eventTypeEnabled:

case MouseEvent.MOUSE_WHEEL:
      if ((eventMask & AWTEvent.MOUSE_WHEEL_EVENT_MASK) != 0 ||
          mouseWheelListener != null) {
          return true;
      }

This returns true if a mouseWheelListener is installed – which is done unconditionally by BasicScrollPaneUI, and not removed when the wheelEnabled property is changed (the ui doesn’t even listen to that property …) Plus the listener simply does nothing if the property is false. At least one of those facts is a bug, the ui should

• either remove/add the listener depending on wheelEnabled
• or: implement the listener such that it dispatches the event up the chain (as Walter does in his example)

The first option can be handled by application code:

scrollPane = new JScrollPane();
scrollPane.removeMouseWheelListener(scrollPane.getMouseWheelListeners()[0]);

it’s a bit of a hack (as bug-workarounds always are :-), production code would have to listen to the wheelEnable to re-install if needed plus listen to LAF changes to update/re-remove the listeners installed by the ui.

Implementing the second option in slight modification (as to Walter’s dispatching) by subclassing the JScrollPane and dispatch the event to parent if the wheelEnabled is false:

scrollPane = new JScrollPane() {

    @Override
    protected void processMouseWheelEvent(MouseWheelEvent e) {
        if (!isWheelScrollingEnabled()) {
            if (getParent() != null) 
                getParent().dispatchEvent(
                        SwingUtilities.convertMouseEvent(this, e, getParent()));
            return;
        }
        super.processMouseWheelEvent(e);
    }

};
scrollPane.setWheelScrollingEnabled(false);