the intention is for you to toggle the mouse-click-passing on... click... then toggle it off.

it is not intended, nor recommended, to try and play while passing mouse clicks. that just won't work for numerous reasons.

as for focus-follows-mouse ... uncheck it and see. with it off, you will need to first click the wow window, then click again to loot. with it on, you mouse over and click once to loot (as the focus was forced to the new wow window as you moused over it)