How to overlay ImageViewer component outline?

Why do we see the outline of the ImageViewer component (see capture) and how to hide it?

You need to use a canvas instead.

Because it’s not actually a viewer control, it’s an input control.

The underlying macOS component is called ImageWell and the whole point of the border is to communicate to the user that this control is for image drop input.

As Beatrix said, use a Canvas to display images. Only use this control if you’re going to accept image input by drag and drop.