viewport rendering - continue rendering and keep resolution when zooming in.

I use viewport rendering to quickly find the best number of samples for the final render. The problem is, when I zoom in to check if the detail is good enough, cycles starts re-rendering so that the area zoomed into now has more pixels covering it, meaning I can't find out if the result was good enough before zooming in.

In the render options there could be a 're-sample on zoom' option which is on by default (to keep the current behaviour), but when disabled, you could zoom in without the render restarting or the resolution changing.