I have the same problem. In fact, it's been a bit of a problem in all the cars I've owned over the past 20 years.

Not so much the waterfall when at a stop so much as while driving. I also smoke and like/need to vent the car while smoking and find that openning the window enough to actually vent allows a lot of water in.

The obvious problem is the integration and stylization of the gutter channel on the roof as part of the fluidity of design. I wish they's make these channels a bit deeper but they dont.

Still, I guess it beats the appearance of those welded-on ribs that ran up and down the roof line on each side like most car roofs of the 60's and earlier.

Better still would be the triangular vent window of the past in an updated and sleeker power version.