Snap map frame vertices to features

[wayfaringrob]

I'm looking for a way to snap the vertices of the map frame to features within the map. In this example, I'd like the edge of the map to be the edge of the riverside parcels - in essence, remove the river from the frame entirely. I wouldn't want it to be permanently linked; that would get tricky quick. Currently, you can only snap to other layout items, and there's no trace mode. The only workaround I can think of right now is either doing this roughly by hand or using external software such as Illustrator.

[AubriKinghorn]

Have you tried using layer clipping in a map? Clipping the map can provide the same visual impact, without requiring you to change the shape of the map frame (which can sometimes have undesirable impacts on graticules, borders, and backgrounds).