I'm not sure if this feature (set icm=split) exists in regular Vim but it exists in Neovim. Here is my own demonstration of that feature: https://www.youtube.com/watch?v=dY9dME3l-iQ
So when that matches window is shown in command-line mode and I press Ctrl-F to edit command in more regular vimmy window css-color is failing with this error: