Variable: ScreenSpace 
constScreenSpace:Component<{bottom: {default:string;type:String; };height: {default:string;type:String; };left: {default:string;type:String; };right: {default:string;type:String; };top: {default:string;type:String; };width: {default:string;type:String; };zOffset: {default:number;type:Float32; }; }>
Defined in: packages/core/src/ui/screenspace.ts:39
CSS‑like screen‑space layout for a PanelUI.
Remarks 
When XR is not presenting, the panel is re‑parented under the active camera and positioned in pixels using CSS‑like expressions for top/left/bottom/right, with width/height specified in any CSS units supported by the browser (e.g. px, vw, vh, %, em).
On XR session start, the panel is automatically returned to world space.
All size inputs are ultimately converted to meters using the camera frustum at zOffset.
Example 
entity.addComponent(PanelUI, { config: '/ui/menu.json' })
entity.addComponent(ScreenSpace, {
  width: '40vw',
  height: 'auto',
  bottom: '24px',
  right: '24px',
  zOffset: 0.25,
})