Klaus,
The screentolayer action is really useful. Unfortunately it has a nasty bug. When a container has a border the coordinate values get messed up. The bug is simple. Instead of subtracting the border width it gets added.
I set up a [url='']demo page[/url]. the top container has no border and works as expected. The one below has a 10px border. Mouse.x and the computed layer.x values are displayed above. Notice as you mouse over the containers and the border how the values change. The problem will be obvious.
I hope you can fix this very soon.
BTW, is there a way to "get" the border width of a container?