CardRenderer

Card -- a visible boundary around exactly one child.

Outlined and transparent rather than filled, which is the implementation guide's own recommendation and not a styling preference. §4 puts the problem plainly: a card inside another card has to stay distinct, and the obvious fix -- alternating surface colours by depth -- means every renderer carries a depth counter and every component below one has to be told what it is sitting on. A transparent container with a 1dp outline nests to any depth by drawing an inner boundary inside the outer one, and costs no context passing at all.

The child is drawn inside a Column holding the guide's 16dp inner padding. That padding is localised in the guide's sense -- it keeps the content off the card's own border and is not part of the outer layout, which is what leafMargin carries.

Material 3's OutlinedCard again, after a spell as a bordered Box. Built on this, a card arriving in a surface update -- replacing a component of another type -- segfaulted Kotlin/Native on macOS and iOS (#31). The fault was never in the card: it was the fun interface implementation behind A2uiComponent's one Render call changing between compositions, and that call is now keyed on the renderer (#98). CardScrollSwapTest makes the same swap with this renderer and, with the key taken off, still dies there -- so it is that test, and not this renderer's shape, that keeps the crash away.

The catalog gives a card exactly one child, and a payload wanting more is told to wrap them in a Column. Nothing here enforces that: the children are iterated, so a payload the schema refuses draws all of what it named rather than silently dropping the tail. CatalogValidator is where a caller that wants the refusal asks for it.