370. A reveal can keep to one axis

Date: 2026-09-17

Status

Accepted. Closes book/src/TODO.md’s “A reveal moves both axes at once” as an option, with the default unchanged.

Context

ScrollController.reveal scrolls the least it can on each axis independently. ADR-0120 argued that is right, and the TODO entry agreed. Its one complaint was that it “occasionally moves a view further than a person would have”: a wide table asked to show a cell slides sideways as well as down. Nobody had asked for an axis to take priority.

Decision

reveal(self, clip, axes) moves only along axes; the two-argument reveal is reveal(self, clip, ScrollAxis.BOTH).

• The distances are computed as before, and the one for an axis axes does not include is dropped before scrollBy. ScrollAxis already names the three choices, so no new type is needed.
• The measurement ahead of a glide (ADR-0363) applies to both axes before the filter.

Consequences

• A caller that only means “bring this row into view” passes VERTICAL, and the horizontal position the user chose is left alone.
• ScrollControllerTest’s harness takes an optional stylesheet, so a grid can be wider than its viewport.