The bordered boxes the sidebar draws, in vertical order. Each Rect is a
complete panel area (border included) or Rect::ZERO when the box is
hidden. Shared by the renderer and the mouse hit-test so click targets can
never drift from what is drawn (the old flat layout used magic row offsets
for exactly this and they drifted constantly).
Split the sidebar area into stacked bordered boxes. Every box is fixed
height and a trailing spacer absorbs leftover terminal height, so the
boxes always stack compactly at the top. When the terminal is too short
the last boxes clip, matching the previous single-panel behaviour.