works with BenQ GL2250 (but not following)

[RJVB]

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This works with my BenQ GL2250 and doesn't show the tendency other implementations have to reset the brightness if you move the slider too fast.

However, following the main screen brightness does not work on my MBP 8.1 under OS X 10.9.5 . When I toggle the option the external brightness is clamped to 0 (fortunately that's not "off") and I have to restart the application after deactivating the follow mode to get control back.