Styling problems for "Docked Window"

As shown in the first figure, the red arrow area is red and white two “Docked Window”, now is red Window blocking the lateral extension of white Window, I want to by some Settings make Bai Seyan "at both ends to the Window, such as the effect of the second picture, I tried many ways, but don’t work, could you tell me the correct solution, thank you.

In the designer, Project Properties -> Vision -> User Interface, set the ‘Axis Precedence’ to North/South: