Resizing the preview doesn't work this way for me. By default I always have crop region checked so I can easily drag the red border lines to the size I need. Just resizing the window doesn't work because the outer edges(usually the bottom) are still way out. I cant see the red cropline border.
Try it yourself, with crop region checked, and set the edges to full size, it doesn't fit after you change the render dimensions and resize the window.
