Color picker for keyer seems flaky - or is it me?

In order to pick a color to key, you can click and drag from the 'picker' box in the keyer properties and drag the eye-dropper over the source window (I'm probably using the wrong terms for this). Anyway - when I do this the image color flashes between green (in this case - it's green screen footage) and transparent but when I release the mouse it often does not 'take'. I'm not in front of my machine at the moment but I wondered if there is some basic mistake I'm making (e.g., I should be using the right mouse button and drag-pick-release instead of the left mouse button...).

Its same here on my windows machine.

I think what is happening is the picker is reading the colour then applying that as the key which changes the BG, then reading that new BG colour and then applying that as a key (usually back to the original) so it creates a loop. You can sample it with out the display updating, I think it is by holding Alt, Shift or Ctrl as you click drag the sampler button.

Thanks Kel - I'll give that a try next time I'm sitting in front of Fusion.

Fusion Tool Manual:

How to Key
Use the Pick Button on the Background Color to select the the blue or green screen color from
the image. Hold Option (Alt) while click dragging the pick and it will pick the color from the up
stream image, making the key not flicker.

Thanks. I know - RTFM. I usually do but I was using BCC chromakey studio and assumed incorrectly that it was a plugin issue. Then I tried one of the built in keyers and noticed the same thing - and that's as far as I took my investigation.