With previews off:

And with previews on:

This desperately needs a better implementation:
- If previews are off, the dialog should shrink vertically to the height of the controls + histogram.
Of previews are on, the dialog should expand vertically to also display the previews.
In no case should the histogram/controls panel change height! The histogram/controls should always maintain a fixed vertical size, with the histogram showing a useful (not too tall, not too short) proportion.