mash: Cursor initially visible on device without touchpad usage

On ToT@#503187 build chromeos with mash for a device (eg. cyan)
(1) Run chrome on device with the --mash flag ( go/mustash-intro )
(2) Run "start ui", careful not to use the touchpad
(3) Observe the screen
Expected: The cursor is *not* shown before using the touchpad (like cash)
Actual: The cursor is shown before using the touchpad

This only seems to repro with --mash, not --mus, but seems MUS-related.

I found this while trying to find the underlying cause of  Issue 756163 .
(I'm still not sure what's causing that mouse move at the origin in mash)
Maybe this cash/mash discrepancy in initial cursor visibility is related?
(please remove my workaround there if it's not needed after fixing this)