CORRECTION

Actually I got that wrong. It is not necessary to call `set_maximum_size' for a workaround. The following code prevents the problem from happening. 

create dialog
box := new_border_box
dialog.disable_border
dialog.disable_user_resize
dialog.put (box)

Ah, I recently changed the order of the call to `disable_user_resize' in my application-wide parent dialog. This must why I am getting this problem now. So it was probably there in 15.01 too.

But in any case, there is inconsistent behavior with GTK so it must be regarded as a bug