For some users, the ZBrushCore window does not properly render on the screen. This usually results in one or more edges of the UI being visible and about half the title bar being cut off across the top of the screen.
There is no fix for this issue at this time.
However, this issue is strictly cosmetic. It does not affect your ability to use ZBrushCore in any way. All the buttons that should be present are still there, even though the top part of some may be hidden.