Intent to ship: InputEvent.inputType (with Input Events Level 1)

[Masayuki Nakano]

Summary: Implement and ship InputEvent.inputType for conforming to Input
Events Level 1.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1447239

Link to standard:
InputEvent.inputType:
https://w3c.github.io/uievents/#dom-inputevent-inputtype
Value of InputEvent.inputType:
Level 1:
https://rawgit.com/w3c/input-events/v1/index.html#interface-InputEvent-Attributes
Level 2: https://w3c.github.io/input-events/#interface-InputEvent-Attributes

Platform coverage: All

Estimated or target release: 66

DevTools bug: N/A

Do other browser engines implement this?

Safari supports Level 2, Chrome supports Level 1, Edge does not support
this.

web-platform-tests:
https://searchfox.org/mozilla-central/source/testing/web-platform/tests/input-events
# And I add a lot of inputType check into existing mochitests.

This attribute is set to the reason why the "input" event is fired. This
is not so important for "input" event but really important for
"beforeinput" event which should be implemented as soon as possible.

The value list difference between Level 2 and Level 1 is, Level 2 adds
some inputType value for IME composition. However, Chrome team decided
that they ship it with Level 1 due to risk management. And I see some
spec issues of Level 2, so, I think that Level 2 is still unstable spec.
So,from points of view of both compatibility and risk management, we
should use Level 1 values for now (but you can switch to Level 2 support
with pref, "dom_input_events.conform_to_level_1", although could be
changed in review process).

--
Masayuki Nakano <masa...@d-toybox.com>
Software Engineer, Mozilla