Cause: a `DockNode::Stack` laid its panels out from fixed heights (Preview
46 pt) with the rest shared, but only split gutters were interactive, so
the right column's panels could be expanded or collapsed and never resized.
Fix: the gap below a stacked panel drags the nearest open panels above and
below it against each other, neither going below 40 pt. The new sizes are
stored as the entries' heights, so they persist in the layout like split
sizes. The last panel without a height keeps sharing the rest, and when no
panel takes the rest, or space runs short, the heights scale so the stack
fills its column. Split and stack gaps share one `gutter` helper.
Automation ids: `dock.gutter.<path>s<i>` for the gap below stacked entry i.
Test: ui_tabs `stacked_panel_gaps_resize_the_panels` (drag, persistence,
collapse/expand, minimum); dock unit tests `stacked_panels_fill_their_column`
and `dragging_a_stack_gap_trades_height_between_its_neighbours`.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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