While bringing up `tests/kicad/load-pcb.spec.ts` (load a real `.kicad_pcb` through
File→Open), three distinct errors surfaced. One is fixed; two remain and turned out to be
**the same underlying disease**: Emscripten Asyncify has a single global suspension register
(`Asyncify.currData` + `Asyncify.state`), but KiCad-WASM has **three independent subsystems**
that all drive it (tool coroutines, modal/clipboard sleeps, and the parked main loop). When any
two overlap, one reads a buffer that no longer belongs to it → **crash** (`index out of bounds`)
or **hang** (a swap unwinds but is never rewound).
## TL;DR
- **The single slot is not a WebAssembly/Binaryen law — it's a choice in Emscripten's JS
runtime.** The Binaryen Asyncify pass is multi-buffer by design; `asyncify_start_unwind`/
`asyncify_start_rewind` take the buffer pointer as an argument.
- **"Give each context its own buffer" is the standard, supported solution** — it's literally
what Emscripten *fibers* are, and what QEMU/TinyGo/Pyodide do. We already do it for tool
coroutines (each `wasm_fcontext` owns a buffer). The bug is that the **fiber path and the
`handleSleep` path both blindly overwrite the one global `currData` register.**
- **We are not switching to JSPI.** The fix stays within Asyncify, in the wasm/shim layer.
- **The achievable universal fix** is a single cooperative scheduler that owns `currData` (and
the fiber trampoline) and treats every suspendable thing — coroutines, modals, clipboard,
fonts, and the main loop itself — as a registered context with its own buffer.
## Document index
| File | Contents |
|---|---|
| [`01-background-and-findings.md`](01-background-and-findings.md) | The originating session, the e2e test + logs, and the three concrete bugs (rtree=fixed; clipboard crash; tool-open hang). |
| [`02-asyncify-internals.md`](02-asyncify-internals.md) | The machine, in detail: what "sleeps", the single `currData`/`state`, the three producers, `handleSleep`/`fiber_swap`/trampoline, the #9153 shim, and full control-flow walkthroughs (startup→park, the hang, the crash, **de-parking** down to the JS line). |
| [`03-solutions-and-prior-art.md`](03-solutions-and-prior-art.md) | Is there a real solution? Per-context buffers, who has done it, why we're not switching to JSPI, and the unified-authority recipe with failure modes. Sources/URLs included. |
| [`04-decisions-tests-open-questions.md`](04-decisions-tests-open-questions.md) | How the fix options relate (what's subsumed vs. genuinely separate), the one diagnostic that decides scope, the combinatorial test matrix, and open questions. |
| [`05-design-a-js-asyncify-arbiter.md`](05-design-a-js-asyncify-arbiter.md) | Incremental design: keep current `EM_ASYNC_JS` sleeps and fibers, but put one JS arbiter in charge of `currData`, transition queueing, and the trampoline. Includes concept explanations. |
| [`06-design-b-fiber-first-runtime.md`](06-design-b-fiber-first-runtime.md) | Cleaner long-term design: make modals, clipboard, fonts, nested loops, and tools all scheduler-owned fiber-like contexts. Explains how this relates to de-parking and app lifetime. |
| [`07-decisions-and-outcome.md`](07-decisions-and-outcome.md) | **What was decided and shipped (2026-06-12):** root cause, red-green ledger, the arbiter NOT built and why, roads not taken with revisit triggers. |
| [`08-dom-port-regression.md`](08-dom-port-regression.md) | DOM-port regression investigation after rebasing onto the async hardening: traces, symbolized crash, ruled-out Asyncify/table/removelist hypotheses, and current stale-window diagnosis. |
| [`09-dom-window-lifetime-hypothesis.md`](09-dom-window-lifetime-hypothesis.md) | Concrete failure story and first fix experiment for the DOM-port stale `wxWindow` hypothesis: destructor ordering, DOM event reentry, and validation plan. |
| [`10-resolution-menubar-uaf.md`](10-resolution-menubar-uaf.md) | **RESOLVED:** the regression was a freed `wxMenuBar` left in a live frame's child list by `wxMenuBarBase::Detach()` (DOM-port only — the bar is a real child there). One-line fix in `wxMenuBar::Detach()`; full kicad suite green, zero corruption signatures. |
