feat: add createSlot fallback option - #21
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A fallback component renders while the slot has never received an insert. The first insert removes it for good: a later clear empties the slot but does not bring the fallback back. The latch lives in snapshot identity: clear now sets a CLEARED sentinel distinct from the initial EMPTY, so the render stays pure and concurrent-safe with zero allocations. Slots without a fallback keep the exact same Root code path. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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What
createSlot({ fallback })— an optional, prop-less component rendered while the slot has never received an insert. The firstinsertremoves it for good: a laterclearempties the slot but does not bring the fallback back. Composes withpresence: true(the fallback is not probed and never counts towarduseCount/usePresence).How
The one-shot latch lives in snapshot identity instead of a mutable flag:
store.clearnow sets aCLEAREDsentinel distinct from the initialEMPTY, andneverInserted(state)is a reference check. Render stays pure and concurrent-safe, zero allocations.Perf
Rootcode path (branch happens once atcreateSlottime); the insert path is untouched.length === 0check perRootrender. No subscriptions, no probes.Tests
12 new runtime tests, 3 type tests, 3 store tests — 152 total, plus tsc/knip/oxlint/attw/publint clean.
One deliberately changed pin: the store test "empty state is the shared EMPTY reference" is split — cleared state is now a stable shared reference distinct from the initial one (that distinction is the feature).
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