Anyway, found this by accident. I forgot to disable the power settings in Windows 10 so when they kicked in the guest window (was at 1920x1080 windowed) shrank to something like 320x240. As soon as I moused over the screen it snapped back to the correct size. Both the 4.3.26 and the 4.3.27 test build have the same behavior.
edit: no longer shrinks build 10049.