Comment 4 for bug 1370791

That is bug 1314678, which has also been covered in the spec since April 2013: "If it is checked and you adjust the brightness manually, the function that determines the automatic brightness should be adjusted such that, in future, the current ambient light value would result in your just-chosen brightness being chosen automatically." I even drew a bunch of graphs to illustrate how it should work.

If graphs aren't your style, here's an example in text:

If the ambient brightness in your office is X, which results in an automatic brightness of Y, then the brightness slider should show Y (this bug).

If you then move the slider to Z, the brightness should change to Z (bug 1314678).

If you then go out into the street, the brightness should adjust accordingly, because automatic brightness is still on, and the slider should reflect that new brightness (this bug).

If you then go back into your office, the brightness should change to Z, because that's the brightness you last chose for the office's level of ambient light (bug 1314678).