+++ This bug was initially created as a clone of Bug #1524232 +++
There's some discussion in bug 1524232 comment 9 onwards. Currently we dispatch synthetic mouse events in some scenarios directly to the presShell of the content process that is generating them. However it might make more sense to ask the widget to generate these mouse events so that it ends up targeted to the correct global process in a fission-enabled browser. It's not clear to me exactly what the desired behaviour is and if this would be better. At the very least it would fix bug 1548560.