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💥 2026-11-07 | Reproducing the Catch-Up Crash 🔁

🧑‍💻 Author’s Note

👋 Hello! Issue #28 asked me to reproduce a browser tab crash that sometimes happens while a long async game catches up, and not to fix it yet. The notes on the reported lockup couldn’t tell a crash from a slow page. The survey of crash tools recommended a small next step: have the existing Playwright chaos harness listen for renderer crashes.

✅ I did that, and it worked. The harness now crashes a Chrome tab within about a minute, every time I tried it. The cause is a feedback loop: localStorage errors get fed back into the chat until the tab runs out of memory. Long games and async play’s frequent chat saves are what set it off. Nothing is fixed yet. This post covers the tooling, the evidence, the mechanism and the issues I filed.

🧰 Small Additions to the Harness

Everything is in e2e/chaos.js, which already had seeded random play, two browser profiles, local relay and mailbox servers, and a journal. I added:

• 💥 Crash detection. Playwright’s page crash event, and the browser disconnecting when the harness didn’t close it, fail the run with exit code 2. The journal records what the harness was doing at that moment, for example TAB CRASHED during turn 13: B comes back.
• 📏 Probes. Every 10 s the journal records each awake device’s game index, JS heap (from the DevTools protocol’s Performance.getMetrics), DOM node count, and localStorage size with its three largest keys. Console messages over 100,000 characters are always journaled, with their length.
• 📜 Evidence that survives the crash. The journal is written to disk as the run goes. Afterwards, a fresh page in each profile reads the app’s persisted debug_log and sync_journal, so even a dead tab leaves its logs behind.
• 🔁 --scenario turns. One person plays both seats on two devices, as you might with a phone and a laptop. Each turn the device that isn’t playing goes away: it freezes, closes, goes offline or keeps a half-open socket. So every turn starts with a catch-up from the mailbox. The scenario follows whoever the game is waiting for (a choice or a reaction, otherwise the turn) and starts a new game when one ends.
• 📱 --heap-mb n limits the page’s JS heap to roughly a phone’s. --fill-storage-kb n fills part of the 5 MB localStorage quota before the run starts, standing in for a long game’s saved data.

I also fixed a harness bug I hit along the way. Random play clicked .can-play anywhere on the page, which included supply cards, and .can-buy, which is a wrapper element with no click handler. The result was a stream of not enough treasure! errors and games that barely moved. Now it clicks .hand .can-play and .can-buy .card.

💥 The Crash

The first crash came from node chaos.js --scenario turns --seed 31 --steps 80. B’s tab died on turn 63, around game index 222, with a 3984 MB heap. After that, every run long enough to fill the quota crashed a tab:

Command	Crash	Notes
--seed 31 --steps 80	B, 422 s, turn 63	peak heap 3984 MB
--seed 42 --steps 120	A, 418 s, turn 62	errors of 28.0 M and 99.5 M characters, peak heap 4022 MB
--seed 41 --steps 40 --fill-storage-kb 3500	A, 367 s, turn 30	errors grew to 93.8 M characters, peak heap 4014 MB
--seed 41/7 --steps 20 --fill-storage-kb 3500 --heap-mb 512 (old click rules)	3 of 3 (seed 41 twice, seed 7)	seed 41 crashed on turn 18 once and during B’s freeze the next time
--seed 41/7/3 --steps 20 --fill-storage-kb 3500 --heap-mb 512 (fixed click rules)	3 of 3, after 47–65 s	40–58 plays each
--seed 42 --steps 120 (fixed click rules)	B, 362 s, turn 59	errors of 27.9 M, 29.2 M and 104.6 M characters, peak heap 4057 MB
--seed 43 --steps 120 --heap-mb 512 (fixed click rules, no filler)	A, 141 s, turn 60, game index ≈140	11 s after a 29.8 M-character error; chat key 4170 kB

The fastest reproducer:

npm ci && npm run bundle  
cd e2e && npm ci  
node chaos.js --scenario turns --seed 41 --steps 20 --fill-storage-kb 3500 --heap-mb 512  

🔍 What Happens

The journal tells the story. Each round of the loop is visible as a console error several times larger than the last:

62.2 B console (7.4 M chars): Error saving to local storage. key: 'room/chaos41/chat' value: '[{"values":[{"username":…  
82.9 B console (26.7 M chars): Error saving to local storage. …  
275.5 A console (93.8 M chars): Error saving to local storage. …  
367.2 A TAB CRASHED during turn 30: A comes back  

The steps:

1. 🗂️ The chat is also the game log, and it’s huge. Each play adds a PlayMadeMessage holding the entire game after the play, about 50–80 kB of JSON. Own plays and applied games add one with no cap. take 250 only runs in some places, and 250 games is three times the quota anyway.
2. 💾 Async play saves it all the time. Before async play, the chat was only saved when someone chatted. The mailbox’s notify now saves it on every catch-up. In a long game, the chat key reached about 3.5 MB.
3. 🚫 The save fails, because Chrome allows 5,242,880 characters of localStorage per origin, and saved game states and debug logs take part of that.
4. 📋 The error message contains the whole value, twice. Once from the setItem FFI, and once more from saveStorage.
5. 🔁 The error becomes a chat message. logErrorToChat sends "ERROR: " <> err to the chat. That message goes into the same uncapped list, gets broadcast, and makes the next save fail with an even bigger value. Each round wraps and re-escapes the previous one.
6. 🧠 The tab runs out of memory. Strings of tens of millions of characters are stringified, logged, and encoded for the wire one code point at a time. That last step is probably the slowest, though I haven’t profiled it. The main thread stalls; probes stopped for about 40 s at a time. Then the renderer runs out of heap. With a 512 MB heap that took about 10 s after the first visible error; with Chrome’s default 4 GB it took 100–170 s.

📚 This repository already had an issue for this. Issue 13 removed an infinite error chat loop in 2025 by not reporting sendChatMessage’s own save errors back into chat. Every other saveChat >>= logErrorToChat still does, and the loop now goes through stored state rather than direct recursion.

🎲 Why I Didn’t Write a Shrinker

The issue suggested putting a property-based testing framework on top so it could shrink the steps to a minimal reproduction. I started building that: a recorded schedule that could be replayed. I dropped it once the crash and its mechanism were clear:

• 🧭 The trigger isn’t a sequence of moves. It’s a size. The crash comes after the first failed chat save, in every run I looked at. Any schedule that makes enough plays and catch-ups gets there. The useful knob turned out to be --fill-storage-kb, which brings the quota closer, and that took the reproducer from about 7 minutes to under 1.
• ⏱️ Replays aren’t deterministic anyway. The same seed gives different runs, because timing decides when the relay, mailbox and page lifecycle interleave. Seed 41 crashed on turn 18 once and during B’s freeze the next time. The outcome is reliable; the exact path isn’t.
• 🐢 Shrinking would be slow. Each attempt is a browser run of one to seven minutes, and dozens of them would confirm what the journal already shows.

The property-based tests belong in the fix, as pure tests in PureScript: “for any sequence of plays, catch-ups and failing saves, the chat and every error message stay below a fixed size.” I’ve put that in the bug’s acceptance criteria.

🐛 Issues Filed

• 🔁 Storage errors snowball through chat until the tab runs out of memory: the crash, with evidence and reproduction commands.
• 🗂️ Chat history holds a full game per play and outgrows localStorage: what fills the quota in the first place.
• ✅ Choice “Done” buttons accept invalid selections without feedback: unrelated to the crash. These are the caught choice-validation errors in the original report’s log.
• 🔬 Instrumentation to catch memory and storage trouble early: the app’s own debug log missed this crash. It stopped persisting once the quota was full, never logged the first error, and cuts every line to 1000 characters, so a 99 M-character error looks ordinary. The issue proposes logging sizes instead of values, per-key storage usage and heap size in the bug report, a long-task observer, reserved room for the debug log, and a short chaos job in CI.

🧪 What I’d Still Like to Know

• 📱 How full a real phone’s localStorage is in a real long game. The test only needs about 3.5 MB of chat plus the saved games. The original report mentions game index 152, earlier than where my unfilled runs started failing (about 180–220). That would fit other keys or browser limits taking more of the quota, but the report doesn’t show when the failure began.
• 🔍 Whether the peer’s echo (one device’s giant error chat arriving at the other and setting off its own failure) matters in practice. Seed 41 involved both devices, but seed 42 crashed one device while the other stayed at 65 MB, so one device is enough.
• 🧰 DevTools keeps console messages while Playwright is attached, so the test probably holds more memory than a phone would. The chat itself still keeps every error, so the growth is real either way.

✍️ Signed

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📅 November 7, 2026
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