Unfortunately, these posts are not numbered, but the "fix" is my post of September 24, 5:05 pm.
I'll restate it here with some clarifications. But this is not the fix for everyone with MBP-HDMI problems, and as I've suggested previously, you may have to get Apple support.
Go to Finder.
In the root directory (MacBookPro or whatever your HD is called), go to Library, go to Preferences, find com.apple.windowserver.plist and put it into Trash
In your own user directory, go to Library (to see the user Library folder in Finder, click the Go menu in finder and at the same time hold the Option key down, and then you'll see Library). Go to Preferences folder within the Library folder, go to the ByHost directory within the Preferences folder, find com.apple.windowserver.[random numbers and letters].plist, and put that plist file into Trash.
There may be more than one com.apple.windowserver.xxxxxx.plist file in both of these directories, including one that ends in plist.lck--just remove any plist file with "windowserver" in it. You won't cause any harm by removing a plist file.
You do not have to empty the Trash folder, but you can if you want to.
Reboot computer.
I don't know how to state it any more clearly than that. If it doesn't work, and it may not, especially with non-Retina MBPs, then you have to call Apple support.