The calculator text runs past the window edge. The window is not resizable to deal with it.
See attached jpg.
Also, when set for hex, it still shows decimal in the result which isn't horrible but does create noise if the user only needs to work in hex. An optional "mixed" mode might be a solution.
Lastly, when directly entering data into the window in "hex mode" the user still has to type 0x for the values or it creates an error. It's not really bad, just a little tedious. This is not a big deal because most users will use this like the Windows calculator and use the button entry method.