Window::Format().setResizable(false) only removes the thick border, preventing you to drag the frame to resize the window. But the Minimize and Maximize buttons are still visible in the title bar and they allow you to resize the window. That's not what you want. This change will remove the icons, too.
Merge pull request #490 from paulhoux/window-prevent-resize

Window::Format().setResizable(false) only removes the thick border...