Unknown reference to GWT widget prevents clearing of detached DOM tree

I've also posted this question in Stackoverflow. Sorry to crosspost, but I wasn't sure of the best place to post.

I've got a GWT 2.4 app where I'm "swapping views" by switching out one Composite widget on the RootPanel for another, using the usual RootPanel.get().clear() and RootPanel.get().add(newWidget) to remove and add, respectively.

The first composite widget contains a PasswordTextBox. It listens for the Enter keypress, which triggers the swap. Nothing too fancy:

getDisplay().getPasswordBoxForKeyPresses().addKeyPressHandler(new KeyPressHandler() {    
    public void onKeyPress(KeyPressEvent event) {
        if (event.getNativeEvent().getKeyCode() == KeyCodes.KEY_ENTER) {
            swapWidgets(); // clear RootPanel and add new widget
        }
    }
});

The problem is that there is a DOM memory leak: after RootPanel.get().clear() is called, the old composite widget is stuck in the detached DOM tree because the HTMLInputElement for the PasswordTextBox has some strange reference to it which I cannot identify.  I compiled at style=detailed and started trying to drive down the tree to look a reference to the element in JS. I'm pretty new with GWT, so it still isn't obvious to me what's going on. So starting with the second line in the retaining tree as in the screenshot below, I can see that lastEvent in _2 contains the nativeKeyTarget listed at the top of tree. But where do I go from there?

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