Ubuntu 10.10
Kdenlive 0.7.8

I have trouble getting the screen look right as this picture (http://www.kdenlive.org/composite-kdenlive-077)... Is it at bug?

My screenshot here: http://www.kdenlive.org/composite-bug

Not sure what your problem is... Composite transition has been changed so that the transition rectangle is now adjustable on the project monitor, see this blog entry:
http://ttill.de/2010/09/12/kdenlive-0-7-8-on-monitor-effects/

You should be able to adjust the rectange directly in the monitor...

@j-b-m: Thx! I'm new to Kdenlive and have been watching a lot of youtube today about Kdenlive.. but apparently the older version ;-)
Did try the monitor - but it seems like I had to make at double-click - now it works.

Thanks!