Experimenting with some secondary colour correction I did something (while sampling a colour) that drew a filled square. Subsequently I removed the plugin but now I have that little square in every clip. If I use colour correction and change saturation to zero (B&W) that square isn't affected by the controls. It seems that I've made an area of every video track that is immune changes and even appears in previous projects (on preview screen). Now I observe that the cursor has changed to a cross and that I can draw wherever I wish rectangles/squares on the preview window, so it seems I've activated a facility new to me. Can't see how to return it to normal, when hopefully the square will go away. Any ideas on how I get back to normal?

Apparently this is a feature of the clipboard and I selected this on the split-screen view button. Click that off fixes the problem (it seems). Interesting. Guess I should refer to the manual.