This is observed from the profile  at bug 1170538 comment 5. Update() is a synchronous IPC which returns an array of EditReply, are there any other ways to avoid it blocking main thread? Will bug 1166173 resolve this or is it possible to interrupt current compositing and redo after RecvUpdate()... Note the app updates "style.transform" of list items to change their position while scrolling.  http://people.mozilla.org/~bgirard/cleopatra/#report=7facf52aabfaa0c48a6d40acbe5e5b0d9c8de353&select=2080,2450
It should be noted that blocking the main thread is considered acceptable, it's generally blocking the compositor thread that's really bad.
(In reply to Bas Schouten (:bas.schouten) from comment #1) > It should be noted that blocking the main thread is considered acceptable, > it's generally blocking the compositor thread that's really bad. I'd say "considered bad, while blocking the compositor is completely unacceptable". Removing synchronous transactions is worthwhile and Bug 1180326 will help with this (it got backed out on b2g because of some breakage but I suppose it will get fixed and reland soon-ish).
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