Rendering Cycles image doesn't pause Viewport render - bug or a "feature"?

When starting an image render in Cycles (Menu Bar -> Render -> Render Image), the rendering starts immediately, but if the viewport is set to rendered, the rendering in the viewport continues too, making both viewport and image rendering take much longer than it should as the CPU or the GPU resources are split between the two.

Is this a bug or a feature? I’d definitely treat it as a bug but I am checking here before I report it.

It is a bit unfortunate, but it also was like that back in the 2.7x series too.