I wonder if its possible to create a semi-transparent (read-only) overlay window on top of an existing window?
My idea is that: If you code in language X that has the possibility to incrementally compile at the same time you could in the overlay window give hints about the code. The reason of it being overlay is of course that you don't want to modify the existing source that the user is editing.
And this is mostly _not_ for errors and such as there is good functionality for that already. The notifications can be pretty much anything. like hinting about if a function may be tagged as "slow", how to reformat a loop to improve performance, etc, etc. This would be kinda cool as you could on a per-line basis update the hints according to the users action.