If the note's $URL is populated, the url tab will preview the linked webpage. The web content in the preview is live and links can be followed (but it is not intended for general web browsing).
Dragging the pop-over will result in a tear-off window that will persist until the end of the current session (i.e. the document and/or app are closed). A torn-off window may be closed during the current session if no longer needed.