Opening a large board could kill the editor runtime outright: "index out of
bounds" / "unreachable executed" / "indirect call signature mismatch", after
which every later focus/key event trapped. Reported in prod against a big
uploaded board, and correlated by the reporter with the moment the presence
WebSocket connects.
That correlation is the tell. Presence/collab work enters the wasm through
runOnFiber -> COROUTINE::Call, i.e. emscripten_fiber_swap, whose stop_unwind
corrupts Asyncify's single currData slot when ANOTHER context is already
parked there. docs/features/async/13 pins the invariant: exactly one
unwind/rewind transition in flight, and prescribes "a single shared is-a-
transition-in-flight guard the pumps consult before re-driving".
Normally the slot is free when fibers drain: the main loop's per-frame
wxWasmYieldToBrowser completes every frame, so drainFibers runs between
yields. It is NOT free when a nested/modal pump tick drives ProcessEvents
while the chain that opened the modal is parked deeper down — precisely a big
board open (progress dialog over a parked load). The wx dispatch interlock
does not cover this: the modal parks deliberately zero its count so their own
pump may dispatch.
So gate the swap itself: drainFibers defers (re-CallAfter) while
asyncifyInFlight(). Bodies are viewport/overlay/apply work, so waiting out the
park costs latency, never correctness.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H8jo7zz1ZwzYpjJ64UZKN4
32218122c3