I decided to investigate today while https://twitter.com/ uses so much CPU when it's displayed. (I'm using a debug build, and in a session with a very large number of tabs, which can both skew things, though probably the oddness is more likely the number of tabs than the debug build nature, although probably 25% of the time is debug build stuff.)
What seems to happen is that we're repainting frequently due to an offscreen animated image, https://abs.twimg.com/a/1415021474/img/t1/spinner-rosetta-gray-32x32.gif , which is always near the very bottom of the page and thus very rarely visible. But each tick of the animated image (which I believe is a background image) sends us through ImageLoader::DoRedraw, which in turn calls nsIFrame::SchedulePaint, so that we do display list analysis to see if anything changed. (Nothing has, since the image is well offscreen.) But constructing and comparing the display lists takes a good bit of time, leading to twitter using a good bit of CPU when it's being displayed. Is it reasonable to test that the image is offscreen (or, really, not in the region that's painted into a layer) somewhere earlier in the process here? Or is there something else we should be doing to avoid this? -David -- 𝄞 L. David Baron http://dbaron.org/ 𝄂 𝄢 Mozilla https://www.mozilla.org/ 𝄂 Before I built a wall I'd ask to know What I was walling in or walling out, And to whom I was like to give offense. - Robert Frost, Mending Wall (1914)
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