Render difference border area

When resizing the border area of the render in the viewport, to render only the remaining part of the border in cycles. So lets say i make an area to render with Shift B, then it renders to 256 samples, then i drag and resize the border, now i want that cycles keeps the existing picture and renders only the new border difference.

Obviously if i change any render parameter, then of couse it has to completely rerender.

The same goes if i make the border smaller, i dont want it to rerender, obviously. (it does now)