"Close" button on "staging files" sort-of-dialog not activatable by keyboard

Screenshot: https://www.dropbox.com/s/sjvj4nf6ripi794/Screenshot%202016-09-29%2012.50.08.png?raw=1

 The close button in the screenshot cannot be activated by the keyboard:

• 99% of the time it's not in the tab order so you cannot give it the keyboard focus
• Enter key does not fire it
• Space key does not fire it
• It has no & keyboard shortcut so there is no Alt-key combination to fire it

With the increased screen sizes and resolutions hitting it with the mouse takes more and more time.

This besides the fact that for mouse-challenged people it won't work at all (hence high priority).

Similar issues likely hold for other buttons as well since SourceTree for Windows does their own sort-of-model-form "dialog" handling.

--jeroen