Slider: can't focus a second time?

[bill]

In test_slider.html on FF2/mac (and maybe other browsers):

1. click the slider handle --> focus goes to slider (slider border becomes black)
2. click a blank area of the screen --> slider blurs (slider border becomes gray)
3. click the same slider again --> slider doesn't focus (slider border doesn't change)

I assume this is a general problem with _base/focus.js, nothing to do with Slider per se.

[bill]

(In [11074]) Fixes #4755: Slider: can't focus a second time