Coverity indicates |TabChild::RecvRealMouseButtonEvent| passes uninitialized value |unused|  to |InputAPZContext| which then stores it in a member and static variable . It's possible this is harmless but it looks like the simplest solution would be to initialize the value to |nsEventStatus_eIgnore|. Flagging as sec as I haven't determined whether this can lead to undefined behavior or not.  https://dxr.mozilla.org/mozilla-central/rev/f7b746b4e91307448cb0746a41f677bfc23908b0/dom/ipc/TabChild.cpp#1930  https://dxr.mozilla.org/mozilla-central/rev/f7b746b4e91307448cb0746a41f677bfc23908b0/gfx/layers/apz/util/InputAPZContext.cpp#45,50
This should be harmless. It was in fact done intentionally - see https://bugzilla.mozilla.org/show_bug.cgi?id=1199885#c101. Is there a way to make Coverity ignore it unless the garbage starts getting used?
FWIW I'm not opposed to initializing it to something if that's the pragmatic thing to do.