Set your app window to the full-screen native display resolution, covering the whole screen. This full-screen mode is also known as "borderless full-screen." Unity renders app content at the resolution set by a script (or the native display resolution if none is set) and scales it to fill the window. When scaling, Unity adds black bars to the rendered output to match the display aspect ratio to prevent content stretching. This process is called letterboxing. The OS overlay UI will display on top of the full-screen window (such as IME input windows). All platforms support this mode.