For now, when running NRWT with --no-pixel-tests, DriverProxy maintains two drivers one of which run non pixel tests and one run ref tests.
This causes problem of chromium-android which doesn't support multiple drivers on one device. On chromium-android we don't use DriverProxy but restart the single driver when pixel-test mode changes. Bug 91533 groups the ref tests together to reduce the cost of restarting the driver on chromium-android.
The problem would be resolved if we can let one driver/DRT support support both pixel and non-pixel tests, that is, the --pixel-tests should be passed as a per-test arg instead of a command-line parameter to DRT.
I believe this has been basically fixed in bug 91754 and bug 92398.
*** This bug has been marked as a duplicate of bug 92398 ***