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How do I move an item above the separator line in the New submenu of Windows 10's right-click context menu?

Is there anything in the Registry that needs to be set?

Create a Config subkey under the ShellNew key of your file type. Then inside that subkey, add a string value named BeforeSeparator. The change should take effect immediately:

To figure this out, I had to dive into the Explorer code. shell32.dll contains a CNewMenu class that represents the New menu and has a function named _BeforeSeparator that determines whether the given entry should be placed before the separator. Here's the important part of that function's disassembly (courtesy of IDA):

First it uses PathMatchSpecW to see if the key name is one of .lnk, .library-ms, or Folder. If it is (red/left path), the item is guaranteed to appear above the line. If not (green/right path), the function checks whether a certain bit (10h) in a certain part (+8) of the data structure is set.

I needed to find the code responsible for setting that bit. There's a function called _GetNewObjectInfoForKey that seems to be responsible for setting up the relevant data structures. Here's part of its disassembly, almost right at the beginning:

It tries to open a subkey called Config and, if successful (red path), uses the _GetConfigFlags function to set the part of the data structure containing the bit we're interested in! _GetConfigFlags goes along testing for the existence of various entries and setting various bits if appropriate. Here's the part that sets our target bit (10h):

And there we see the desired entry name: BeforeSeparator.