Digital Light & Color

Help states:

Histogram Mode
You turn on the histogram mode by clicking the button on the main tool bar. Activating the histogram mode pops up a histogram window. What is displayed within the window depends on the current image and the settings.

Issue :
It is unclear that in a transformation, the current image is the output image displayed in the preview window. "The histogram is always displayed for the output image."

Help states:

Markers
When displaying histograms, clicking or dragging anywhere on the input image will display a red marker line on the histogram at the location in the histogram that corresponds to the color of the image where you clicked. This is helpful in understanding what part of the histogram different parts of the image contribute to. Markers accumulate as you click or drag over the input image and are cleared when you release the mouse button. For RGB histograms, red, green and blue marker lines are displayed. For 2D hue-saturation histograms, the affected color patches are drawn in red.

Issue:
"The Histogram Tool probe is no longer active when a transformation is open and you click on the input image. "

If you read farther down in the help topic:

Input/Output Image Histograms

When a transformation dialog box is open, the histogram is computed from either the input or the output image. If the display tool bar at the top of the dialog box is set to show just the input image, the histogram is computed from the input image. Otherwise the histogram is computed from the output image. This lets you toggle between viewing the input and output histograms by clicking on the display tool bar, by pressing the Ctrl-D keyboard shortcut or by Shift-clicking on the Amount control mask box. When no transformation dialog box is open the histogram is always computed from the output image histogram.

I had forgotten about this feature -- for the next release, I restored probing the input image, but only if the screen is not split.