meaningful, since the resulting distance will never be less than
@code{padding.}
+@c TODO: explain skylines somewhere and xref to it from here.
+
@item
@code{padding} -- the minimum required amount of unobstructed
-vertical whitespace between two items, measured in staff-spaces.
-This can be thought of as the minimum height of an unobstructed
-(invisible) rectangle that extends from the leftmost to the
-rightmost point of the combined items.
+vertical whitespace between the bounding boxes (or skylines) of
+the two items, measured in staff-spaces.
@item
@code{stretchability} -- a unitless measure of the dimension's
@item
@code{padding} -- the minimum required amount of unobstructed
-vertical whitespace between the two items, measured in
-staff-spaces.
+vertical whitespace between the bounding boxes (or skylines) of
+the two items, measured in staff-spaces.
@item
@code{stretchability} -- a unitless measure of the dimension's