Chrome version:
  Windows: 71.0.3578.98 & 73.0.3642.0
  CrOS: 73.0.3654.0 (pixelbook & coral)

to reproduce:
(1) get a device with a touchscreen
(2) visit this page: https://rbyers.github.io/eventTest.html
(3) open a new window (any will do) so it partially covers the eventTest window
(4) interact with the eventTest page via the touchscreen
(5) under Windows & CrOS, the eventTest window will automatically be raised to the foreground.  in both cases, the touch events are being correctly tracked as seen in the debug log.
(6) click the config button and enable preventDefault for touchstart
(7) move the new window back so it partially covers the eventTest window
(8) repeat step (4) and observe Windows behaves the same, but on CrOS, the window remains in the background.  in both cases, the touch events are being correctly tracked.

this behavior (window receives events but is not focused) is atypical for CrOS.  webpages are at a loss for rectifying the situation as there are no web platform APIs (that i know of) to force the window to be focused.  which means webpages are unable to fully capture & customize touch event behavior without also changing the behavior of focusing.  not that they should have to work around this in the first place.
there are some Chrome extension APIs, but those don't work on the open web.

tl;dr: CrOS should raised the window that is touched even if the page is capturing the touchstart event & calling preventDefault for it.