In the image provided and as per your comment I understand that you are not able to see the Add selected button in the modal window. I tried to reproduce the same in my 8.3 with Appstudio. Kindly find my inferences below.
In Chrome, I was able to see the add selected button in modal window in both case of maximized /minimized window.
In IE,I was able to see the button in modal window with maximized screen.However,in case of reducing the size, I was not able to see the button.
In this case only I am able to replicate the issue of not seeing the button. And also once the focus is in the modal window when trying to press Tab to get the focus after the last widget, I am able to see the buttons.I guess the buttons were not seen in ViewPort.With trying to get focus on the button,some calculations are happening after which these buttons comes up in screen.
Kindly let me know if we are seeing the issue in all browsers or In IE like the above case. If issue is only in IE, try getting the focus to buttons with Tab key and see if they are coming up. If not then then the buttons are not in screen.