Post Ps CS Color Picker issues

As of Photoshop CS (I believe) my windows filter plugin's color picker no
longer works correctly.

The picker comes up, and the dialog controls, including the buttons, radio
buttons, and numeric fields function correctly. Clicks on the client area
of the picker, including the square color area and vertical slider, just
generate a beep.

If I go back to earlier versions of Photoshop, the picker works fine - it's
just for CS and later. Any thoughts or guesses are welcome.

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