Here's the mojom used for transport from window server to app:
https://code.google.com/p/chromium/codesearch#chromium/src/components/mus/public/interfaces/input_events.mojom&q=input_events.mojom&sq=package:chromium&l=57

It promises screen coordinates, but it's unclear if they really are screen or not. In particular, if you have a secondary monitor to the left of your laptop's primary monitor, those events need to have negative X coordinates.

(Right now in ash_sysui and chrome browser, these coordinates are stuffed into ui::Event::root_location_: https://code.google.com/p/chromium/codesearch#chromium/src/mojo/converters/input_events/input_events_type_converters.cc&sq=package:chromium&type=cs&l=378&rcl=1462202263

This is wrong, "root_window" should be relative to the root window of the target. It happens to work in ash code because the root window has the same bounds as the display. But that's a separate issue.)

Regardless, it's not clear if "screen" in that code means that same thing that "screen" means in existing ash code.

I think we should use the current definition of "screen" coordinates in ash for mojo transport.