Recreate: click the new file button in the toolbar. In the dialog, uncheck "add to project", click OK. The new file "Untitled<#>" opens, with the cursor at the correct spot, but the tab for the new file is darkened such that it appears you're editing another tab in the window. See attached screen shot.
This also behaves the same if you add it to the project and give it a name.
The editing takes place in the correct file, and if you click the tab, it moves to the foreground and then behaves normally.
This was present in v18 as well.
I can't repro it on the Mac.
-Tim