A column rule ran from the top of the columns to their bottom, so a frame
that the text only half fills drew its rules down to the empty bottom of
the frame. InDesign draws them from the top line's ascent to the bottom
line's descent. column_rule_rects now takes that extent from the frame's
lines, and the top and bottom insets shorten the rule from there. A frame
without lines draws no rules. Existing documents draw their column rules
at the new length, on screen, in PDF and in PowerPoint export.