Pausing by default can fail due to missing presContext

[simonlindholm]

E.g. if you visit http://existential-elevator.tumblr.com/ and then navigate to page 2, the gifs there (which seem to live within an iframe that gets cached, or something) get animated despite the "Pause GIFs by default" setting. The reason is that

getDwu(win).imageAnimationMode = playing ? 0 : 1;

fails when content-document-global-created runs, because the window lacks a presContext at that point.

We might be able to force a presContext, e.g. through win.matchMedia("(min-width: 1px)");, but somehow it doesn't seem to work, but anyway that's a hack and it's bad for performance and memory.

The only real remaining option is to set the global image.animation_mode pref, and restore it on uninstall.

[simonlindholm]

E.g. if you visit http://existential-elevator.tumblr.com/ and then navigate to page 2, the gifs there (which seem to live within an iframe that gets cached, or something) get animated despite the "Pause GIFs by default" setting.

That reason is apparently different - at the point when content-document-global-created fires, we haven't yet gotten the right document. document-element-inserted appears to work much better. So this issue is less serious than I thought.