361. A column is resized by asking

Date: 2026-09-17

Status

Accepted. Closes book/src/TODO.md’s “A table has no column resizing”.

Context

docs/design-system.md §3.1’s table row says “column resize: 1:1, like split-pane’s drag”. The TODO entry proposed a split-pane between the headers, but a split divides one box between two panes and a table has any number of columns, some fixed and some weighted.

Table is a stateless leaf, and everything about its rows is the application’s: the order, the selection, and the widths through Column.fixed and Column.weight. Sorting already works by asking: a header click reports what the sort would become, and the application hands back rows in that order.

A 1:1 drag has to be measured from the width the column had when the drag started. For a weighted column only the layout knows that.

Decision

A resizable column’s header carries a grip, and a drag on it asks the application for a width in pixels, anchored at the width the header last came out as.

• Column.resizable(true) and Table.resized((key, width) -> …). The application answers by making the column fixed(width); a weighted column that is dragged becomes a fixed one.
• A resizable header is built inside TableHeaderCell, a stateful composition node with no CSS type, which keeps the header’s last laid-out width through Measured. The header answers gestureAnchor() with it.
• table-grip is a 6px part over the header’s trailing edge, positioned right: -12px because an absolute box here is placed against the content edge and §3’s cell padding is 12. It handles the drag: on each move it asks for anchor + dragX, with a floor of 32px so a column cannot be dragged shut. The router asks the pressed chain deepest-first for an anchor, so the grip reads the header’s width. It consumes the press, release and click, so a drag never sorts.
• table-header loses overflow: hidden. That clips at the content box and hid the half of the grip over the padding; the label still clips itself.

Consequences

• The table goldens are unchanged: a column that is not resizable builds no cell node and no grip.
• A resize is a rebuild per pointer move, the same cost as a split-pane drag.
• There is no keyboard resize. A header is focusable only when it sorts, and a second meaning for its arrow keys is a decision for when something asks.
• The showcase’s Name column is resizable, with its width kept in the card’s state; gallery-collections is re-blessed for the caption that says so.