** some extra work needed to land pieces separately
** high cost of merging with other changes to the code
** trying to land automated tests for bugs reported after the previous landing
** [https://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?changeset=b2aeb8be3ded first piece] and [https://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?changeset=a5ee3b2e133a second piece] landed
* pieces of off-main-thread animations landed / landing: {{bug|706179}}, {{bug|755084}}