Reviewers

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Bugzilla Bug ID

1913666

The fullscreen-painted observer topic is named after its original implementation detail which is a paint observer inside FullscreenTransitionTask. But every external consumer (site-permission panel, WebAuthn prompt helper, MacTouchBar, test harnesses) seem to use it as a "fullscreen transition has settled" barrier. The "painted" guarantee is already best-effort: FullscreenTransitionTask falls back to a 1s timeout (full-screen-api.transition.timeout) and continues regardless. Over time the topic also acquired four manual re-broadcast sites (DOMFullscreenChild on MozAfterPaint, two paths in DOMFullscreenParent, and pictureinpicture/player.js against the PiP window) with subtly different triggers.

This patch does not rename or remove the topic, but the redesign collapses the redundant emit sites and removes the dead content-process broadcast and its parent→child Painted IPC loopback; with hopefully the result being that the Fullscreen IPC architecture rework, will out of the box provide the same side-effects in the UI, as the JSWA implementation does.

In the future, we may want to audit this and possibly remove it entirely.