Fix regression on IE10 clicking blank screen to close drop down.

An e.preventDefault() call on MSPointerDown prevents the button from getting focus,
so I stopped doing it for that case.  This is a minimal fix.  Other approaches include:

  1. remove the e.preventDefault() call for all cases
  2. make focus manager listen for touch/mouse events in the capture phase rather than the
bubbling phase, to avoid missing the event due to a stopPropagation() call; it used
to do this except for IE6-8, but I normalized the behavior in 9965406.

Fixes #17262.  Refs #13048, #13341.