Sorry about the lack of information! I have tried your solution and it does not work for me.
Let me give you the whole picture, and hopefully we can come to some kind of resolution.
I have a screen, with around 10 kendo windows in. We have a header (top: 0, height 45px) and a footer (top: window height - 45, height 45).
We have custom functions for resize, dragend, maximize, minimize and restore. This is so we can handle the window management ourselves. (For instance, when we maximize, we check if the window has a kendo spreadsheet inside, and we resize the spreadsheet to fit the entire window here).
Our maximize function will maximize the window to the whole viewport size, minus the height of the header and footer.
Inside our header we have a kendo menu, (as shown in the screenshot).
Whena window is maximized the list items of the menu (second level ones) are shown behind the window. I tried with your solution but it doesnt work.
First screenshot - shows the menu items with a higher zindex than the windows.
Second screenshot (the window is maximized, I have just moved it) - you can see the menu items are behind the window, and behind the grey area (body).