Turns out, not a #Chrome bug instead a #WebTestRunner / #Puppeteer bug. Apparently concurrent tests run multiple tabs in the same window by default. This means only one page is visible and pages in the background don't update consistently, meaning `requestAnimationFrame` just doesn't get called sometimes.

https://github.com/modernweb-dev/web/issues/2588

This is fixable by giving each test its own window, so every test is always visible.

Definitely not an obvious workaround though, so hopefully this is something which can be fixed by default.

Pretty convinced I've got a Chrome bug on my hands here at this point.

Took all night, but I was able to narrow down the test cases to two files <15 lines each failing on latest #WebTestRunner and #Puppeteer.

Last step is to strip #Jasmine from this and see if I can reproduce similar behavior with `setTimeout`, then file the bug.

I've tried to get a minimal reproduction but this _literally_ takes at least 6 tests across 2 different files to replicate. Disabling any _one_ test fixes all of them. No shared state between them which I can find. The two files *should* be completely isolated from each other anyways (using #WebTestRunner).

I'm completely baffled here.

@westbrook I *think* I got all the options. I don't see anything else in your example which looks related.

https://github.com/dgp1130/composite-weak-map/commit/49c09375ec9550e9a0328b0ef8250783f2407c40#diff-24d782e02f46a1eb18aed719d1c73354c23a5702d6e324a6163f6d90a6965cad

Even outside #WebTestRunner I can't get it to work. Just running the #Chrome binary directly with those flags doesn't work either. There must be some other condition for `window.gc` I'm not finding...

@westbrook Ooh, that's very cool. I was not aware of that API.

I'm trying to copy what you did there and was able to spawn #WebTestRunner with `measureUserAgentSpecificMemory`, but for the life of me can't get `window.gc` to be defined. I'm setting the `--expose-gc` flag just like you seem to, but I can't get Chrome to start with it. Is there some other special incantation I need to do this?

https://github.com/adobe/spectrum-web-components/blob/e5a1a61bae448005abab9e6aa1a4be5c3efa3372/web-test-runner.utils.js#L37

@VamptVo Sounds like you're looking for #WebTestRunner?

https://modern-web.dev/docs/test-runner/overview/

Building on this area...

Testing is like a magic power, but lean into magic realism over magic fantasy for better results: https://dev.to/westbrook/testing-with-js-is-like-magic-but-is-it-science-23f8

Now to get to work on the next installation!