Was using an older build of mirc for a while I forgot the build number but was at least 2 years old.
Due to letsencrypt certificate changes, I updated to the latest version to get the new bunder certificates, and up on restarting mirc I can now see there is almost no separator between the text box and the chat window,
I see in the changelog notes, in some recent builds there is some changes related to colours and borders, I think one of them may have regressed things.
I use a black background for both the text box and the chat box. If its relevant.