Bug 78186 #8 from Simon Fraser (smfr) 2012-12-16 15:04:31 PST: There's a problem with this strategy, when combined with zooming;when zoomed in, we may consider a fixed element to be outside the viewport, and not composite it. However, zooming may bring that element back into the viewport, but if it doesn't have a layer yet, it won't be visible while zooming.