See bug 1312770, comment 53. The difference stems from when this code was first landed. I'm not sure if I had a reason for that, but having these two pieces of code disagreeing is not what we want.
Pushed by email@example.com: https://hg.mozilla.org/integration/mozilla-inbound/rev/86e46eaaacfb Consider visibility: hidden frames to be not visible in nsIFrame::UpdateVisibilitySynchronously to match PresShell::MarkFramesInSubtreeApproximatelyVisible. r=mats
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