Keep a non-default slot's content when its sibling default slot flattens

[nathanacurtis]

Problem

A component with a default slot and a second ordinary slot loses the second slot's content once the library declares a default-slot convention.

Before the convention is declared, both fills are captured — each as its own slot-content entry, referenced from the nested instance. After it is declared:

• the default slot's fill correctly flattens into the host's own anatomy, elements and layout
• the ordinary slot's fill disappears from every concern of the generated spec
• no warning names it

ADR-099 is explicit that both shapes coexist: a fill into a non-default slot keeps binding through an explicit reference while a default-slot fill nests as plain children, side by side in one spec.

Potential solution(s)

• Flatten only the default slot's fill, and keep every other slot's fill as the explicit reference it already was
• If a sibling fill genuinely cannot be carried alongside a flattened one, warn and name the slot rather than dropping its content

Acceptance criteria

• A two-slot component whose default slot and ordinary slot are both filled keeps both fills in the generated spec
• The default slot's fill is nested as plain children; the ordinary slot's fill stays an explicit slot-content reference
• Content is never dropped without a warning naming the slot it belonged to
• A component with only a default slot, or only ordinary slots, is unchanged

Case data

• Fixture: testSlotNonDefaultNested, testSlotTwoSlots
• Workspace: specs-testing — round-trip testing while verifying MVP: Derive the default slot from the api concern's prop, matched by prop name #670
• Territory: specs-from-figma
• Size: m

Notes

Reproduced by declaring slots.default.match: [children, items] in the workspace's spec conventions and regenerating both fixtures:

• testSlotTwoSlots gains defaultSlot: true on children and nothing on aside — correct
• testSlotNonDefaultNested then captures the children fill as nested anatomy (root > levelZeroText > level1 > twoSlots > aChild) and loses the aside fill, which previously held a text element

A render round-trip of the resulting spec is otherwise clean and byte-identical in structure, so this is a capture-side loss rather than a render one.

This is the anatomy-capture flattening route, not the ADR-076 container-promotion route. The slot detector's default-slot suppression documents the intent correctly — it skips only the default slot and keeps descending so a non-default slot is still found — so the cause is not yet identified and no line is claimed here.

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Implementation details are tracked internally.

[nathanacurtis]

Fixed in specs-from-figma. All four acceptance criteria are capture-side and all four now pass.

The rule, as settled

Only a defaultSlot: true slot may be consolidated into a host's own layout tree. Every other slot binds through the instance's props to its own slotContentExamples entry, always. So the registry shrinks rather than disappearing.

The cause

Not an oversight — a deliberate gate. Instrumenting the pipeline showed the fill was detected correctly and the wiring relation was created, and then the pass that stamps the pointer never ran:

[slot]  name=aside enclosingInstance=twoSlots containingSlot=none isDefault=false
[push?] aside -> anchored=true
(no wiring pass)

Crossing a default slot re-enters the host's own space, so aside no longer has a containing slot to be addressed through. That moves it from the route that works for a component onto the route that was switched on for compositions only. Captured, registered, unreachable — and unreferenced entries never surface, so it vanished silently.

The gate's stated reason was that nobody had decided whether a slot on an instance inside a component's own layout is composable from outside. The rule above answers that, so the gate now opens whenever the default-slot convention is in force. The pass-through regression it guarded against is still prevented where it belongs — the wiring never overwrites a configuration an element already carries.

Result on the fixture

# variants.yaml — the tree consolidates only default slots
layout: [root: [levelZeroText, level1: [twoSlots: [aChild]]]]
# …and twoSlots binds the rest
twoSlots:
  propConfigurations:
    aside: { $slotContent: "…/slotContentExamples/testSlotTwoSlots__aside__default" }

# examples.yaml — three entries became one, not zero
slotContentExamples:
  testSlotTwoSlots__aside__default: { … content: Aside }

Verified at scale

Workspace	Before	After	Content
specs-testing (the fixture)	3 entries, Aside lost	1 entry, Aside bound	preserved
eg egdsCard	57 entries, 2224 lines	53 entries, 2169 lines	preserved
de-library deCard	2635 lines	2635 lines	byte-identical

deCard is unchanged on purpose: its binding chain comes from container promotion, not from this route — that is #665.

One expected consequence on eg: a shared entry referenced from two places is inlined at both once consolidated, so a content string can legitimately go from one occurrence to two. A tree cannot share a subtree by reference; that is the trade ADR-099 makes.

Tests: 2060 passed across all five packages, no existing assertion edited.

Render does not yet consume the shape

A round trip through Figma shows the tree rendering correctly and the bound non-default slot not being filled, so capturing the result loses Aside again. That is a render-side gap, not a capture one, and it is filed separately.