Be sure to be using the latest BIOS posted here:
Then install the latest video driver here:
If the issue persists, test using only the display connected to the HDMI port. Additionally, you may need to test a direct connection between the HDMI port and another display as the issue may be caused by the adapter/converter that you are using.
did the updates but still get partial window when dragging from one screen to the other
Please send me your system configuration, operating system, video cables that you are using and video adapters.