Without this change the only way to close the modal overlay is to click the hide button. Setting the button to default allows to activate it with the ENTER key.

Before:
<img width="849" alt="screen shot 2018-03-06 at 15 14 23" src="https://user-images.githubusercontent.com/3534524/37040276-58af9ce0-2151-11e8-8c55-50acdea669d9.png">

After:
<img width="848" alt="screen shot 2018-03-06 at 15 12 41" src="https://user-images.githubusercontent.com/3534524/37040294-650d1c9c-2151-11e8-8245-2da250a71b3d.png">

make check
bitcoin-qt
# Hit Enter key once the progress modal appears. The modal should hide.