Hi Dragan

>I'd rather go with a red dot appearing at the topleft of the control. If a semi-transparent image would work, that would be even better.

Now that makes a lot of sense! It's a simple, unambiguous, visual cue that does not require anything too fancy and which will work in any UI that I can think of. If fact, now that I think of it I've actually seen this one a few times recently...your work? <g>