In various versions of MacOS, ZBrush (or ZBrushCore or ZBrushCoreMini) may launch in a state where the UI is magnified, displaying the bottom corner of the full ZBrush UI. (See attached image example)
*This is a known issue linked to a MacOS Application setting called "Low Resolution Mode"
Follow these steps to configure ZBrush to launch with full UI visibility:
1. Close ZBrush
2. Navigate to Applications\ZBrushOSX 2021\ (Or whatever the name of your installation directory is.)
3. Right click on ZBrush application > Get Info > Enable option "Open in Low Resolution Mode"
4. Launch ZBrush
5. ZBrush will now display full UI visibility (MacOS will retain this setting for future usage with ZBrush)