Digital Light & Color

I really like the facility available on a lot of transforms of being able to hold the shift key and click and release in the Amount Mask box to see before and after versions in the preview window. This means that I don't have to shift my gaze from the image to toggle between before and after versions.
On the other hand, the 2 and 3-zone Adjustment transforms have a before and after button and I have to look between the buttons and the preview window to see before and after versions.
Is there a way in the 2 and 3-zone Adjustment transforms to see before and after versions without taking my gaze from the preview window?
Mike.

You could Apply the transformation and then compare the input and result windows Either side by side, overlapping, or using Window Blink.

While not exactly what I think you wanted, there also is an off-label use of Composite that may help: use one version of the image as Base, the other as an Overlay, and shift-click on the Amount button at any zoom to compare the two.

Thank you Jonathan and Maciej for your suggestions.
I was hoping that there was some keyboard based method for toggling A(fter) on and off, such as Ctrl A, or Alt A. Or some way of toggling between B and B such as the keys A and B (a bit awkward) or the forward and reverse arrow keys.
It seems not.
So I'll morph my question into a suggestion!
Mike.