Draggable: Fix spurious blur in dialogs on mousedown

I was running into a problem with a popup menu control in a dialog; clicks
weren't working (but keyboard was working fine). It turned out that the menu
was getting destroyed before the click event could fire.

Tracked down the issue to the way draggable blurs focused controls; it was
doing the blur before it ran through the logic to figure out if the drag was
actually on the handle. I've moved the blur below these checks, so it'll only
blur things if it actually needs to handle the drag. Otherwise, it asserts no
opinion on what should and shouldn't be focused, which seems like the way
things ought to be.

Also, added a unit test to check for the expected behavior.

Fixes #15046
Closes gh-1730

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