360. An affix stays inside its container

Date: 2026-09-17

Status

Accepted. Amends ADR-0119. Closes book/src/TODO.md’s “A pinned affix is not pushed out by the next one” and the sticky-header half of “A table has no column resizing and no sticky header”.

Context

An affix pinned itself to its viewport’s edge once it would have scrolled past, and stayed pinned for as long as its hole was above the edge. Two sections with a header each overlapped: the first header stayed pinned while the second arrived under it. The TODO entry said doing better “needs an affix to know about its sibling, which is a relationship nothing in the widget tree expresses”.

CSS answered this without siblings. A position: sticky element is confined to its containing block, so a section’s header leaves with its section and the next header takes over by the same rule. What an affix lacked was the rectangle of the box it is in.

The table’s header was blocked on this. It sits above the table’s list, so a table inside a page that scrolls lost its column names, and an affix there would have pinned them over whatever came after the table.

Decision

Located also reports the painted rectangle of the nearest ancestor with a box, and an affix never travels past that box’s far side.

• Located.located(self, clip, container) is a default method that calls the two-argument form. PointerRouter.notifyLocated finds the container by walking up from the element to the first ancestor that has a hit-test region, because a composition node has no box. With none it reports the window. The router only notifies again when the container moves.
• AffixState limits its shift to the room between the hole’s far side and the container’s: for top, the container’s bottom minus the hole’s bottom; for bottom, the hole’s top minus the container’s top; and likewise for the horizontal edges. :affixed still comes on as soon as the affix lifts.
• An affix directly in the scrolled column has the whole document as its container, so it pins exactly as before.
• Table wraps its head and rule in an affix. At rest the header is where it was; on a scrolling page it pins while the table is in view and leaves with the table.

Consequences

• A sticky header per section is two columns, each holding an affix and its rows, and needs no code.
• The table’s box tree gains affix and affix-content above table-head. The table goldens are unchanged.
• TableHead’s hit testing goes through the affix’s translate, which the router already inverts (ADR-0068).