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.