1. With Firefox on Android
2. Go to https://cloud.google.com/appengine/
3. tap on the top left 3 bars icon to open the menu.

Sometimes the menu open with content,
sometimes without content.

You may have to try a couple of times.

See the diagnosis of Thomas in
https://webcompat.com/issues/19363#issuecomment-513335979

Emilio said:

We may want to delay animationend events as well for example, maybe... It's weird to setTimeout(foo, 1000), starting an animations of 2s duration, and getting animationend before the setTimeout.

Thomas' diagnosis is fairly accurate - it appears they simply coded in an assumption that a particular timeout would occur before a particular other event. The web specs make no such guarantee; they merely say the order timeouts will fire in, and that they will be after the target time. This is in fact a webcompat bug of Google's.

Karl, can you get this to reproduce? I've tried about 10 times over 2 days, and possibly the site was redesigned?

Yes the code has been totally changed.