Quote Originally Posted by Elroy View Post
I also searched for an API call setting (tristate), and there doesn't seem to be one.

It might be possible with some subclassing (or a create window hook), but I'm not motivated enough to test that.
Probably a SendMessage with BST_INDETERMINATE
https://learn.microsoft.com/en-us/wi...ectedfrom=MSDN

Sets the check state of a radio button or check box.
Sets the button state to grayed, indicating an indeterminate state. Use this value only if the button has the BS_3STATE or BS_AUTO3STATE style.