When using the margin size of "small tablet" or "tablet" and in split screen mode, the resource which is on the bottom half of the screen displays a button with "..." on the right of the "book panel controls".
This button is quite difficult to hit when configured as explained here. The semicircle that is used to configure panel linking and to move the panel size takes up almost all of the pixels available for the "..." button.
When configured as "phone margins" there is no problem.