Making the panel re-appear in the lock screen after removing it from the shield

[Jonathan Dieter <jdieter gmail com>]

I'm writing an extension that will basically turn the shield into a
slideshow.  Because I've disabled screen blanking, the shield is on all
the time.  I've put the following code in screenShield.js, into the
_activateFade function to disable the panel when the shield comes down,
and it works perfectly.

Main.layoutManager.removeChrome(Main.layoutManager.panelBox);

The correlating code to bring the panel back is *not* working, but does
not leave any warnings or errors:

Main.layoutManager.addChrome(Main.layoutManager.panelBox, { 
    affectsStruts: true,
    trackFullscreen: true });

I've tried putting it in _clearLockScreen and _completeLockScreen, but
the panel never re-appears.

Is there somewhere else I should put it?  Is there a better way to make
the panel disappear?

Jonathan