ADR 0067: A track spans what the field holds

Status: Accepted

Context

A slider has to span something to be drawn at all. Where a document declares no range, the contract takes the one a bare <input type="range"> assumes — 0 to 100 — and says so in nativeConstraintAttributes, “which is why it is decided here and not left to each renderer to remember”.

Both renderers remembered it anyway, separately:

packages/plain/src/fields/text-field.ts     offered().max ?? 100
packages/lit/src/components/slider-field.ts this.max ?? constraints.max ?? 100

And the default turned into a misrepresentation. A form holding 150 with no bound declared drew a track ending at 100 and put the thumb there:

	the form holds	the page shows	aria-invalid	said
no bound declared	150	100	false	nothing
step: 5	7	5	false	nothing

The second row is the platform snapping a range input to a multiple. Neither is a rule: the document declared no bound, and the validator vocabulary has no step, so there is nothing to appeal to and nothing said. A person sees a slider at its maximum and sends three times that.

ADR 0066 closed the declared-bound half — a value past a declared max is now refused with a message. This is what is left where nothing was declared.

Decision

A track spans what the field holds. sliderTrack(constraints, value) returns the range a slider is drawn on: the bare-input default where nothing is declared, widened to include the value.

Widened only where nothing was declared. A document that declared max: 50 keeps it. The attribute is the native guard and must not promise less than the rules it came from (VAL-004), and since ADR 0066 a value past it is refused with a message — so the page explains the difference rather than hiding it. Where nothing was declared there is no rule to explain anything, and showing the value is the only honest answer left.

A step that would move the thumb is dropped. step: 5 with a value of 7 draws at 5, and there is no step rule to appeal to. The affordance gives way to the value: an increment is a convenience, showing the number the form holds is not.

One source for the drawn fill and for the attributes. Both renderers take the track from the same function, and Lit’s element offers it as its own narrowing so the projection and the painted fill cannot disagree.

Consequences

A slider with no declared bound and a large value now draws a track that reaches it, so the thumb moves as the value changes instead of sitting pinned at the end. The drawn range is no longer a constant a stylesheet or a screenshot can rely on.

step disappears from the attributes while the value is off-grid and comes back when it is on one. A keyboard user arrowing from an off-grid value moves by 1 until they land on a multiple.

nativeConstraintAttributes and MdyFieldShellA11yOptions take an optional value. Both are additive; a caller that omits it gets the previous behaviour, which is correct for every kind that does not draw a track.

Template order became load-bearing in Lit. A range input clamps its value to the bounds it carries at the moment of assignment, so the value must be set after the part applies min/max. It is stated where it matters, because the failure is invisible in review: the attributes are right, the value is right, and the rendered control is wrong.

Alternatives rejected

Widen the track even where a bound is declared. It shows the held value everywhere, and it makes the native max promise less than the rule — the drift VAL-004 exists to prevent — for a case ADR 0066 already explains with a message.

Clamp the value into the track. A widget that rewrites what a form holds is what ADR 0029 forbids, and it would turn a display problem into data loss.

Leave the default and document it. The document declared no bound, so there is no sentence to show a person and nothing they could act on.

Keep step and accept the snap. It is the current behaviour, and it means a slider shows a number the form does not hold whenever a value arrives off-grid — from a prefill, a draft, or a scripted write.

Verification

• packages/widgets/test/form-shell.spec.mjs — the track for a declared bound, an undeclared one, an off-grid step and an empty field.
• battle-tests/browser/a-slider-that-shows-a-different-number.spec.ts — both renderers, with a value inside the range as the control: a repair that widened everything would pass the red and fail that one.
• examples/plain/panels/states.js — the track printed beside what the form holds, in the panel the browser suite drives.

Security and privacy

None. Nothing is retained or transmitted that was not before: the value is unchanged, and only the range it is drawn against moves.