365. An overflowing tab strip pages from its ends

Date: 2026-09-17

Status

Accepted. Closes book/src/TODO.md’s “A tab strip scrolls, and has no chevrons at either end”. Adds ScrollController.position() and onChange, and a CHEVRON_START mark.

Context

A tab strip’s headers sit in a horizontal viewport (ADR-0118), so a strip wider than its window can be reached by the wheel or by dragging the thumb. A tab bar conventionally also has a chevron at each end. The TODO entry named what was missing: a chevron “needs to know it is at an edge, which is scrollIntoView’s missing question again”. A ScrollController could move a viewport but could not say where it was.

Decision

A controller reports its viewport’s position and says when it changes, and a tab list that overflows puts a tab-pager at each end that pages through it.

• ScrollController.Position holds the offsets, the overflows and the viewport’s size, with overflowsX(), canScrollLeft() and canScrollRight(). position() answers NONE with nothing attached. onChange(Runnable) sets a single listener, because a controller has one owner. ScrollState notifies it when the offset moves (by any route) and when its measured extents change.
• TabsState listens and keeps the last position. TabList builds [rule, pager, viewport, pager] while the headers overflow and [rule, viewport] otherwise.
• A pager pages by 80% of the viewport’s width, never less than 40px, through the controller, so the move glides (ADR-0363). It is :disabled at the edge it would page past, is a BUTTON named “Earlier tabs” or “Later tabs”, and is not a Tab stop: the strip is one stop, and a selected tab already reveals itself.
• Box.Mark.Kind.CHEVRON_START is CHEVRON_END mirrored, a kind rather than a transform for CHEVRON_UP’s reason.

Consequences

• The pagers take width from the viewport, so a strip near the threshold has two stable states: overflowing with pagers, or fitting without. Which one it is in depends on the side it was approached from, and it does not oscillate.
• gallery-basic-narrow and gallery-icons-narrow are re-blessed: their strips overflow and now show pagers.