372. A tab is dragged, and the strip asks where

Date: 2026-09-17

Status

Accepted. Closes book/src/TODO.md’s “Nothing reorders tabs”, as an option.

Context

The TODO entry: “a reorder would need a different animation from an arrival: a tab that moves has two positions and nothing to interpolate between them, which is ADR-0097’s missing geometry again. §5 does not ask for drag-to-reorder; the model shape would take it without a change, since the strip draws the list it is given.”

Two things make a drag possible without that geometry. The dragged tab is the only one that needs to move smoothly, and it follows the pointer 1:1, which §1.7 asks of every drag and which needs no interpolation. And tabs already report their painted rectangles through Located (ADR-0120), so the strip can tell where among the other tabs a drop landed.

Decision

Tabs.onReorder((value, index) -> …) makes a strip’s tabs draggable. A drag draws the tab at the pointer, and a drop asks the application to move the tab to an index among the others.

• TabDrag is a composition node with no CSS type around each header of a reorderable strip. It hears the pointer after the Tab, which consumes only the click. Past 4 points of travel along the row a press is a drag: each move reports the travel and the pointer, and the release reports the drop.
• TabsState keeps every header’s painted rectangle while the strip is reorderable, in strip order, dropping rectangles for tabs that have gone. The drop index is the number of other tabs whose centre is before the pointer. Nothing is asked when that is where the tab already was.
• While dragged, the Tab carries a dragOffset that its restyle turns into a translateX, so the tab is drawn where the pointer took it and its slot in the row stays put.
• The strip does not reorder anything. On the drop the tab returns to its slot, and the application’s new list puts it in its new one on the next build.
• A press and release that did not travel still select.

Consequences

• The other tabs jump to their new places when the application’s list changes. Animating them is still ADR-0097’s missing geometry.
• There is no markup: two values do not fit a valued action. The showcase’s chapter strip, which is Java for the list’s sake already, is reorderable.
• gallery-navigation is re-blessed for the caption that says so.