Dev-only Chrome extension that dispatches synthetic message.new and reaction.new events at a Stream Chat Channel for stress / perf testing the chat UI. Triggered from a popup; runs entirely in the active tab.

Not part of the npm package. Not shipped to users.

git clone git@github.com:GetStream/stream-chat-js.git
cd stream-chat-js
# the extension lives at:
ls dev/burst-simulator-extension/

Then load it in Chrome (or any Chromium-based browser supporting Manifest V3):

1. Open chrome://extensions.
2. Toggle Developer mode on (top right).
3. Click Load unpacked and select the dev/burst-simulator-extension/ folder.
4. (Optional) Pin Burst Simulator to your toolbar — easier access while testing.

After editing any file in the folder, click the refresh ↻ icon on the extension's card on chrome://extensions. No build step is required.

Browser support. Chromium-based browsers with MV3: Chrome 88+, Edge 88+, Brave, Arc, Opera. Not tested with Firefox.

1. Open a page where a Stream Chat Channel is mounted (any React app using stream-chat-react works — no manual setup needed).
2. Click the Burst Simulator icon. The status line should resolve to Ready · <cid> · …. If detection fails, the ↻ button re-runs detection.
3. Configure:
   • Total events — synthetic events to dispatch (default 1000)
   • Rate per second — pacing; 0 = fire all in one tick (default 75)
   • Reactions ratio — 0–1, fraction of events that are reactions (default 0.25)
   • User pool size — distinct fake users to rotate (default 10)
4. Click Fire burst. Live progress streams to the status line: Running · 230 / 1000 · 76 eps · 3s. The button morphs into a red Stop — click to abort and get a partial result. Closing the popup does not stop the run; reopening re-attaches.

On click, the popup uses chrome.scripting.executeScript({ world: 'MAIN' }) to inject simulator.js into the active tab's main world (where the page's window and React tree live). The simulator:

• builds a pool of fake users
• generates message.new events with varied text (3–50 words, occasional emojis / fake URLs / line breaks) so the renderer has real layout work to do
• generates reaction.new events targeting any of the burst-generated messages, picked at random; pre-applies latest_reactions / reaction_counts / reaction_scores / reaction_groups to event.message so channel_state.addReaction lands them correctly
• routes each event through client.handleEvent({ data: jsonString }), exercising the WS frame's JSON.parse cost
• paces dispatch by ratePerSec, or fires the whole burst in one requestAnimationFrame tick when rate is 0
• returns { dispatched, durationMs, messages, reactions, aborted } and console.logs the same; the popup also surfaces it in the status line

Note on parse cost. The simulator currently runs an extra local JSON.parse(jsonString) to mirror the pre-#1729 connection.onmessage shadow parse. With #1729 merged, prod does a single parse — flipping the simulator to single-parse is a planned follow-up.

The popup auto-binds the channel by walking the React fiber tree on open. It looks for any prop whose value duck-types as a Channel (getClient + cid + id + type) — including the conventional channel prop on <Channel channel={ch}>. When multiple Channels exist (e.g. a ChannelList of previews next to an active <Channel>), it picks the one whose owning fiber wraps the largest subtree.

Manual override: set window.streamChannel (canonical) or window.channel (alias) before opening the popup, and the fiber walk is skipped.

1. Open the page, open DevTools → Performance.
2. Open the extension popup, set your config.
3. Click Record in DevTools.
4. Click Fire burst. Wait for completion.
5. Stop recording and inspect the flame chart.

For a single all-at-once burst, set Rate per second to 0 — every event lands in one frame, captured as one long task.

• manifest.json — MV3 manifest, scripting + activeTab only
• popup.html / popup.css / popup.js — popup UI and executeScript wiring
• simulator.js — core simulator, runs in the page's main world

• message.new and reaction.new only — no typing, edits, deletes, threads, attachments, or mentions.
• Reactions only target burst-generated messages (not pre-existing channel state).
• Single-channel only (whatever is currently bound to window.streamChannel / window.channel — auto-bound or manually set).
• Auto-detection requires the page to be a React app and the Channel to be live in the fiber tree. For non-React hosts, set window.streamChannel manually before opening the popup.
• Stop is best-effort. In rate per second: 0 (burst-all) mode the entire batch lives inside one requestAnimationFrame tick, so the abort flag never has a chance to be observed mid-run. Use a paced rate if you want to abort.