I do not mean the auto functionality. Sometimes there is no horizon on the picture. In that case, I want to draw my crop bounding box, grab the image outside of the bounding box with the left mouse key and staighten it by moving the mouse up or down, until it looks good for my eye. A combined crop+straighten functionality in one. Like this:
The canvas is rotated under the bounding box. The bounding box size is adapted according to the available size while keeping the aspect ratio.