I don't know if this is a really a bug. But GroupBox in dialogs have a bigger size inside the border of the GroupBox.
I have made a dialog, which works great in 7.78. But in 7.79 there is this problem.
Maybe not a huge issue, but got surprised that my button was overlapped.

Button in 7.79 is overlapped. But in 7.78 it's not.

in mIRC 7.79:
[Linked Image from i.ibb.co]

in mIRC 7.78:
[Linked Image from i.ibb.co]