google/tracing-framework — pull request #371 by chihuahua (2013-07-12 00:03:40+00:00)
Title: Removed irrelevant canvases. Fixes #370.

Previously, canvases were never removed while seeking backwards,
which could have misled the user into thinking that contexts were created
before they actually were. It could have also confused users into thinking
that more contexts were created than in reality. Now, a contexts is
removed if a previous step is sought that lacks the context at
the start of the step.