Unsolved fixed sized window not working in tiling window manager in linux

[saber]

i made a widget that is fixed in size and will show up at bottom of screen. it works fine in Stacking_window_manager
but not in Tiling_window_manager like i3wm in linux. the window manager scratches the window .

but should be like this

my code is

// set window size
    int x = static_cast<int>(CPrime::InfoFunc::screenSize().width()  * .8);
    int y = static_cast<int>(CPrime::InfoFunc::screenSize().height()  * .7);
    this->setFixedSize(x,y);

how can i fix that.