So here is the tip: If you want additional space in your ZBrush UI to put more UI elements, you can actually 'Slide' any part of the UI in the same way you slide the palettes from the left or right tray:
Ctrl key + Click and drag
To slide the UI in ZBrush, hold the Ctrl Key, then click on an 'empty' area and drag to slide that section.
This is pretty convenient if you just want to add a few more buttons to your UI and space is running low... this is why I got some 'blank' spots on my custom UI (so that I can easily click an empty area):
.avif)
