positions, one for above the staff, one for below).
In each pass, we loop through the unplaced grobs from left to right.
- If the grob overlaps the right-most affected position, we place it
+ If the grob doesn't overlap the right-most affected position, we place it
(and then update the right-most affected position to point to the right
edge of the just-placed grob). Otherwise, we skip it until the next pass.
*/