Visually indicate the annotation under the cursor

The current behavior of the interface is to prevent the creation of a new annotation when the cursor is inside the time interval covered by an existing annotation. For long enough video clips, and when the timeline is not zoomed enough, there may be not enough visual resolution for distinguishing between the positions of the cursor for two consecutive frames. It may be frustrating for the user, who may think that she/he is already outside an annotation, but the Return key does not work (for creating a new annotation which, for instance, is intended to be contiguous to the previous one).

One solution for that is to change the visual appearance of the annotation when the cursor is inside the time interval covered by that annotation. For example, the border of the rectangle representing the annotation in the timeline can be made wider in that case.