I did this when I got a new 1080p monitor/TV and it worked fine, but the problem reappeared after installing the latest version of Win 10. The reg hack still works to solve the underscan, but when I start the machine, the monitor no longer picks up the signal. My older 4:3 monitor, which use as a second screen, still works ok though.
As the 1080p monitor/TV is my primary screen, I really need to fix this. Strangely, if I reboot, the screen will work, but not from a cold start.
Just for reference, this eventually solved itself. I tried connecting the monitor using VGA, but it still wouldn't pick up a signal on a cold boot, but would work if I then rebooted. After living with this for a few days, it suddenly started working! Now switched back to HDMI, and that is working ok too. Windows / ATI must have worked out something was wrong and fixed it.
Actually, I spoke to soon. Now what happens is that if the monitor goes into stand by, there's no signal when it wakes up and I only get a picture on the secondary monitor. The only way to get the primary monitor back is to re-boot.
Please can somebody help with this, its driving me nuts!