When editing (and refactoring) a large chunk of code, sometimes it could be really helpful to have the equivalent of a red-pen, or yellow-highlighter, to temporarily mark up bits of the code that need looking at. 

This is very easy to do when printing it, but awkward when working with the electronic file (it's rather ugly to embed lots of "FIXMEs" and then remove them later).

My suggestion:

1. A highlighting mode, which makes the cursor-select give the text a (configurable) luminous-yellow background.

2. These highlights are ephemeral - they don't change the file, and go away when the editor is closed. 
 (they may need to persist if the editor should crash).

3. On close, the editor should warn if any highlights are still present.

4.  Right-click menu on a highlighted-section allows you to quickly remove the highlight.

Thanks for your time and consideration - I hope this is a useful idea.