Browser extension

Look at a tab by hand

A Chrome extension for the case where you want to point at a running app rather than drive one from a script. It is not published to the Chrome Web Store — build it and load it unpacked.

Nothing is instrumented until you say so

The extension ships no host permissions and no declared content scripts. Enabling a site requests that one origin, registers a document_start content script for it, and disabling unregisters the script and hands the permission back.

A leak tool has no business patching WebCodecs on every page you visit. An earlier version declared content scripts on <all_urls>, which patched the globals and exposed the census API in the main world of every page — for a tool you only ever need on one app. That was the wrong default however useful the tool is.

Revoking the permission from Chrome's own settings also stops the content scripts: the extension reconciles stored origins against granted permissions on install, on startup, and whenever a permission is removed.

Two modes

The exact one is not free, so you choose.

Patch mode

The census runs as a world: "MAIN" content script at document_start, and rewrites new Worker(url) to load a small blob that imports the census first and the real worker second. No debugger, no banner, and it coexists with an open DevTools window.

It is genuinely the weaker half, and it says so rather than pretending otherwise:

• A page whose CSP omits blob: from worker-src/script-src cannot create a blob worker at all. The patch catches the throw, falls back to the original URL so the app keeps working, and records the skipped worker with its reason. The popup lists them.
• Chrome does not promise a MAIN-world content script beats a page's inline scripts.
• A worker already running before the patch installs is never covered.
• self.location inside a wrapped worker becomes the loader blob URL. Workers using import.meta.url are unaffected; workers building paths from self.location are not.

Exact mode

The same mechanism as the CDP driver, driven through chrome.debugger so it works on a tab you are already looking at: auto-attach pauses each worker, a beforeScriptExecution breakpoint gives the second, complete pause, and the census goes in there.

It costs a "debugging this browser" banner, and it cannot share a tab with an open DevTools window — Chrome allows one debugger client per tab.

Exact mode injects the plain census rather than the Worker-rewriting build: CDP reaches workers on its own, so the rewrite would be redundant and would change self.location for no gain.

Build and load it

npm install
npm run build:all      # then load extension/dist unpacked

In Chrome: chrome://extensions → enable Developer mode → Load unpacked → pick extension/dist. Manifest V3, minimum Chrome 116.

Using it

1. Open the app you want to measure and click the extension icon.
2. Enable on host — Chrome asks for that one origin. Reload the tab, so the census is in place before the app builds its decoders.
3. Optionally Enable exact mode, then reload again so workers are caught at startup.
4. Refresh in the popup to read the census.

The popup shows, per context: live counts by type, media elements and how many are stalled, anything garbage collected without close(), the top allocation sites with two stack frames each, and anything the install could not instrument. In patch mode it also reports how many workers were wrapped and which were skipped, with the reason.

What it can see about the page

The census API is a page global — window.__webcodecsCensus — readable by any script in the document, including third-party ones. Everything it exposes is already derivable in-page, but treat it as visible rather than private, and do not enable it on a page handling data you would not want an analytics script to see. Allocation stacks contain your source URLs, function names and line numbers.