Nearly all WMF wikis load TimedMediaHandler and on 'BeforePageDisplay' it adds module 'mw.PopUpMediaTransform', which depends on Spinner.js, an earlier version of fgnass's code. TMH uses it to render a fairly large "loading" animation over media players.
It would be nice to only use one spinner library. bug 55550 is to reduce resource code loaded by TMH and MwEmbedSupport. A lot of extensions roll their own spinner, including I think ApiSandbox, Flow, LiquidThreads, UploadWizard...
BTW WMF still has a lot of code using (window.)injectSpinner()
and removeSpinner
from wikibits.js, even though it's deprecated.
This post was posted by SPage (WMF), but signed as S Page (WMF).