We should just check the from value of the new transition instead of
checking computed style. That's because the new transition moves back
to the original position while we are waiting for the callback of paints.
For example, on Android, thumbnail creation is sometimes processed between
paints and callbacks. (Gecko's main loop spins after paints process, most
of callbacks are processed there)
MozReview-Commit-ID: JyJsmEuIiKn
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extra : rebase_source : 80d1f6500ae2ba93ea034c622e59576ef60d09bb
7b8ed2e786