215. A property the engine drops is a rule that does nothing

Date: 2026-08-29

Status

Accepted. Fixes a rule shipped in ADR-0214 and closes a TODO.md entry open since ADR-0109.

Context

table-head shipped with border-bottom: 1px solid var(--gb-border), which §8’s subset does not have: there is one border and no per-edge longhands. The engine did exactly what it promises — logged at debug and carried on — so the line §3’s metrics row asks for under a table header was never drawn, and the golden image was accepted with it missing.

This is the fourth time. TODO.md has recorded it since ADR-0109: “margin is not in §8’s subset, which tab-new found after border-bottom and currentColor. Three properties a widget reached for and did not find, all silently ignored — the subset is right to be small, and nothing warns when a declaration is dropped.” menubar documents the same wall in a comment in the stylesheet itself.

The engine’s behaviour is right and is not what needs changing. A stylesheet naming box-shadow before it is implemented should not stop a window opening, because that stylesheet may be an application’s. What was missing is that the toolkit’s own sheets were being held to the same lenient standard as a stranger’s.

Decision

The rule is a node. table-rule is a box one pixel tall with a background, which is separator’s answer to the same problem and the only one the subset allows. Not a new border-bottom property: per-edge borders are a change to the box model and to Yoga’s edge handling, and one table’s underline is not the case that should decide it.

The toolkit’s own stylesheets are linted, and an application’s are not. SupportedPropertyTest resolves every rule the catalog and the showcase ship through the real ComputedStyle and fails on anything it reports as unsupported. The asymmetry is the point: leniency is for stylesheets the toolkit did not write.

It asserts the behaviour rather than a copy of it. There is no list of supported properties in the test to drift out of step with the engine — it attaches an appender and reads what the cascade actually said. A property added to the engine tomorrow needs no edit here; one removed is caught the same day.

It lives in :example. That module already ships logback, so capturing the cascade’s own output costs no new dependency on :widgets; and §14 makes the gallery the visual regression corpus, which is where a declaration that draws nothing is a screen that has been photographed wrong.

Custom properties are excluded. --gb-accent: … reaches the same branch and is logged the same way, and it is not a fault: custom properties are the resolver’s, computed for var() substitution before ComputedStyle sees a declaration (ADR-0049). Every theme is nothing but those, so the unfiltered check reported 158 failures on a healthy tree — which is how the filter came to exist rather than being foreseen.

The check checks itself. A third test feeds it border-bottom and asserts it is caught, because a change to the log’s wording or to the appender wiring would otherwise make the other two pass by seeing nothing at all.

Alternatives considered

• Adding border-bottom to the subset. It is the fourth request, which is an argument for it — and it is a change to the box model rather than to a parser: Yoga takes per-edge border widths, but the painter draws one stroked rounded rectangle (ADR-0064), so a single-edge border is a different drawing and not a different number. Worth doing when something needs an edge the subset cannot fake; a 1px node is not a workaround here so much as what a rule is.
• Raising the log from debug to warn. It is still a log, and the entry this closes says why that is not enough: one line per property per stylesheet is a message, but it arrives at start-up on a stream nobody is reading, and the three previous instances all had it.
• A hand-written set of supported properties, checked against the sheets. Cheaper to write and the failure mode is drift — a property added to the engine and not to the list makes the test fail on a healthy tree, and the reflex fix is to edit the list rather than to question it.
• Failing the engine on an unsupported property in a toolkit sheet, by marking sheets as trusted. It puts a test’s concern in the runtime, and it would turn a cosmetic mistake into a window that does not open.

Consequences

• One more test guards a whole class of mistake, and it found nothing else: border-bottom in table-head and padding-bottom in the showcase’s #peaks were the only two live instances in the tree.
• :example has logback on its test classpath, where it previously had it only at runtime.
• The four table goldens changed, because the rule is now drawn. The difference between the accepted image and the corrected one is a single line of pixels — which is how a missing rule survives review, and an argument for the check rather than against the golden.
• An application still gets silence. The TODO.md entry is narrowed rather than closed: what warns is a test over the toolkit’s sheets, and an author writing margin in their own stylesheet still gets a debug line and a declaration that does nothing.