[Affinity Designer: measure line/path length]

As of version 1.8.5.703 there is a window called "Transform" that will display the width and height of a selected object.

It also gives the X and Y position relative to the ruler, so you could drag the 0 point of the ruler to the side of object one, and select object two, the X or Y will give you the distance between these two objects.

It is limited in that a line drawn on an angle will not show it's true length nor show radius of a circle, but it's a start 😊