mash: Incorrect window gets click event when dismissing a menu

On ToT @ #545527 chrome os on linux desktop
(1) Run chrome --enable-features=Mash
(2) Create two overlapping browser windows.
(3) Show the app (three dot) menu on the top window.
(4) Click outside the menu, somewhere on the top window.
Expected: The menu closes and the top window is still in front.
Actual: The menu closes and the bottom window is brought to front.

If this doesn't repro, try opening the menu in the other window.
(not sure why, but it seems to repro one way, but not the other)

Seems like something is wrong with event targeting.
Is this a bug in the Window Server? It doesn't repro with Mus.