ManualCrop thumbnails are stretched so that crop effect is not clear

Currently, visible width and height of manualcrop widget thumbnails are set to 100px, not depending on real image proportions:

It seems the problem is in the CSS declaration:

.manualcrop-style-thumb .manualcrop-preview img {
  margin: 0px;
  max-width: 100px;
  max-height: 100px;
}

.manualcrop-style-thumb .manualcrop-preview-cropped img {
  max-width: none;
  max-height: none;
}

max-width & max-height do what they are asked to: limiting image size to the values of 100px. It's obvious that to let an image to keep it's aspect ratio only one dimension must be constrained:

Simple patch provided.