Basically, what I said I strongly suspect was happening in https://bugzilla.mozilla.org/show_bug.cgi?id=1040226#c0 (but wasn't in that case), is actually happening now with different STR: 1. Open http://people.mozilla.org/~bballo/apz-overscroll/index.html in the Browser app. 2. Zoom in. 3. Overscroll horizontally. Actual results: You get stuck in overscroll. Expected results: You do not get stuck.
The simple fix of changing the call to CancelAnimation() in NotifyLayersUpdated() to CancelAnimationsForHandoffChain() runs into lock order problems. I could work around them, but the refactoring in bug 1039992 will make them go away, so unless someone other than me runs into this problem, I'll wait until bug 1039992 is fixed before fixing this.
I can't reproduce this bug in the absence of overscroll-during-panning (bug 1055361), so I'm going to de-prioritize it until we have STR with shipping features.
Let's reopen or file a new bug if/when we have STR with shipping features.