Crash report: https://crash-stats.mozilla.org/report/index/b9886dd7-85a8-4562-a354-7857d0210316

Reason: EXC_BAD_ACCESS / KERN_INVALID_ADDRESS

Top 10 frames of crashing thread:

0 XUL mozilla::layers::AsyncPanZoomController::OnDoubleTap gfx/layers/apz/src/AsyncPanZoomController.cpp:2901
1 XUL mozilla::layers::AsyncPanZoomController::HandleGestureEvent gfx/layers/apz/src/AsyncPanZoomController.cpp:1189
2 XUL mozilla::layers::AsyncPanZoomController::HandleInputEvent gfx/layers/apz/src/AsyncPanZoomController.cpp:1128
3 XUL mozilla::layers::InputQueue::ReceiveInputEvent gfx/layers/apz/src/InputQueue.cpp:79
4 XUL mozilla::layers::APZCTreeManager::ReceiveInputEvent gfx/layers/apz/src/APZCTreeManager.cpp:1746
5 XUL {virtual override thunk} 
6 XUL nsChildView::DispatchAPZInputEvent widget/cocoa/nsChildView.mm:1943
7 XUL nsChildView::DispatchDoubleTapGesture widget/cocoa/nsChildView.mm:2115
8 XUL -[ChildView smartMagnifyWithEvent:] widget/cocoa/nsChildView.mm:2771
9 AppKit AppKit@0x1875a2 

Just encountered this random crash on macOS 10.14 while using the double-tap to zoom functionality on this page

I'll try to further investigate and to isolate some reliable steps to reproduce.

Hmm, we get a double tap gesture while there is another input block that is not a touch block active? So perhaps if the user pans on the touchpad and then quickly double taps we could hit this. I'll try to reproduce and put up a patch.

We can either get here via touch events interpreted as a double tap, or a double tap event sent from the os. The former will have a touch block, the latter will have no input block associated with it.

The crash shows it's possible to get here with another type of current input block.

The other option would be to ignore the double tap if we get here and there is a current input block that is not touch. Not sure if that is better or not.

I couldn't reproduce.

https://hg.mozilla.org/mozilla-central/rev/496c75dc1301

As mentioned in comment 0, this issue occurred randomly. Unfortunately, I couldn't isolate some reliable steps to reproduce.

Removing the qe-verify+ flag but I think it's worth keeping an eye on the graph for a while.