On the Brushes panel (Designer and Photo), the scrollbar is not displayed by default.
When you use a Wacom stylus you can't scroll inside the panel to select the desired brush. I have to use the mouse in the Brushes panel to display the scrollbar.
The current fix I found to display the scrollbar is the activation of the scrollbar on the general settings of macOS but it's for everything on my computer and I would like to avoid that.