sl0p: clear the stale picker region when the layout re-flows

Adding a pane left old borders on screen until a resize forced a full redraw.

Two causes. The picker draws each entry as its own chunk, so the space between
and around the entries was never painted and kept the previous frame's
contents; and adding a pane really does re-flow the whole grid, because a
freshly created pane has no title yet and its "Pane #N" fallback is far wider
than a named one, which widens every cell and drops the column count.

Separately, refresh_or_dissolve_stack_list changed the reserved row count
without forcing a clear, so when the list got shorter the visible pane moved up
and the rows it vacated kept their old contents.

Blanks only the spans no cell covers, never a cell's own columns: chunks are
NOT composited last-wins, so blanking the whole region erased the entries
instead of the stale contents under them -- the first attempt at this fix drew
exactly one entry.

stack_list_row_gaps is a free function so the covering invariant is testable:
gaps and cells must tile each row exactly, with no overlap, across a sweep of
row caps, label widths and member counts.

0 files changed, 0 insertions, 0 deletions