I want to archive the following behaviour of my controls:

I got 2 Controls. the one on the left side contains a dataGrid with 1 column, the one on the right side a bunch of other controls.
When i now klick a part of the left one (the green arrow) i want it to expand over the right control and add some columns to my datagrid.
Problem is: i dont know the width of the right component as it fits to the window-width.
Name:  start.jpg
Views: 128
Size:  44.4 KB

Name:  stop.jpg
Views: 137
Size:  56.4 KB

anyone got an idea how i can archive this?