From the provided screenshot, the left navigation section is over the fixed center panel section. How about collapsing the left panel? The setting 'User resizable' option on the left navigation section can be used for this. This will creating collapsible left navigation panel. Will this meet your requirement?
In OOTB, when the window is restored then width is reduced then the behavior is as per your expected behavior. With only difference being that the center section is not behind.
With the suggested change, the left panel is over the center expanded once user clicks on the UI resizeable. however, when the window is in maximized state, it does not collapse when user takes the focus away from left panel.
Refer the attached video. I hope this will help you to see the behavior at my end.