The combination of tab opening animation + the tab toolbar getting smaller during the customization transition is a recipe for disaster, performance-wise. We should try to wait for the tab open animation to finish before running the customize mode transition. Cc'ing phlsa since he's working on alternatives to the current transition. Until we get an agreed-upon spec though, I think we should continue to work as if the current transition is the desired one.

Here's a try push that delays the transition by 250ms: Baseline: https://tbpl.mozilla.org/?tree=Try&rev=fa48d6ebc486 Patch: https://tbpl.mozilla.org/?tree=Try&rev=fec4ec25fccf Compare talos: http://compare-talos.mattn.ca/?oldRevs=fa48d6ebc486&newRev=fec4ec25fccf&server=graphs.mozilla.org&submit=true Note that a huge spike in the ".error" measurements is expected, since I don't believe CART is prepared to deal with transition delays, so it'll think the transition is taking extra long.

WONTFIXing as we're going to go with the route in bug 979031.