I bit the bullet. After some thought, I decided that it was better to present the options in a pane below the card preview, rather than have the slide over and back. The reduces the number of mouse clicks from three to one to see the result of the chosen option. This brings it into line with the controls to the right which are as instant as I can make them. A much better experience, I believe.
Next – clean up the presentation of the thumbnails (they all have to occupy the same vertical space now), remove some buttons and look at what happens when the window is resized (which isn’t great at the moment).