What steps will reproduce the problem?
1. Place a dock on a border between screens
What is the expected output? What do you see instead?
The dock's space is not accounted for, because the strut is set at the
maxim extent over all screens in the relevant direction.
The size of the root window is the bounding rectangle of all screens, not
the size of the screen the dock is on. This means that if we try to make
space for a dock that lives "between" two screens, it will actually be
allocated on the last screen in that direction.