The new test button seems to work good and will make it easier to adjust offsets properly, however the MkII seems to have a mind of its own sometimes. I made an adjustment to my WB mode (h offset) and tried the test button. It looked right but I pressed Esc to see if it would return me to the old mode, which it did. I then pressed Apply to set the settings I just tested, and then my monitor turned off. Now it doesn't turn on at all in that mode even after a reboot, so it seems that part of the settings are corrupted somewhere as they should be valid. This weird behavior was also there with the previous core.
Another weird issue: I had PPaint on a PAL screen and WB on a HighGfx screen and could switch between them fine. I then dragged down the PPaint screen to reveal the WB screen and then my monitor turned off as it didn't receive a signal anymore. I still had the mousebutton pressed and tried pulling the window back up, but I never got a signal back, even after pressing the button in the upper right corner. I had to reset the machine to get a picture again. I've had other similar issues with the screen suddenly turning off, as if it's ending up in some invalid state it can't get out of, short of a reset.