The crash occurs when dispatching a user input event which is a default action
of a raw user input event like `click` event caused by `mouseup` event if
the raw event's `isTrusted` is set to `false` accidentally during dispatch.
User input events are fired in the main process first. Then,
`EventStateManager` sends it to remote process from `PostHandleEvent()` if
necessary. However, at this time, `WidgetEvent::mFlags::mDispatchedAtLeastOnce`
is never rest, but its only referrer, `EventDispatcher::DispatchDOMEvent()`
assumes that when it's `true`, `WidgetEvent::mFlags:mIsBeingDispatched` is
`false`. Therefore, only in content process, `mouseup` event's `isTrusted` is
set to `false` by `EventTarget.dispatchEvent()` even while it's being dispatch.
And also the trusted state will be used for creating next event which is part
of the default action.
https://searchfox.org/mozilla-central/rev/6566d92dd46417a2f57e75c515135ebe84c9cef5/dom/events/EventDispatcher.cpp#1121,1126,1130-1131,1135,1138,1143
Therefore, this patch makes `WidgetEvent::mFlags` reset `mDispatchedAtLeastOnce`
when it's copied across process boundary and make
`EventDispatcher::DispatchDOMEvent()` won't modify being dispatched events for
avoiding any odd issues.
Unfortunately, this patch adds "expected: FAIL" to the new WPT test only on
Windows. The failure reason is still unclear. I cannot reproduce the failure
on my Windows environment, but on Try Server, it fails permanently since
the driver succeeds to send the mouse click, but the button never receives
`mouseup` nor `click` event.
Differential Revision: https://phabricator.services.mozilla.com/D52988
--HG--
extra : moz-landing-system : lando
Masayuki Nakano