fix: ui sizing in screen-based XR sessions

Author: bbohlender

packages/uikit/src/components/fullscreen.ts

@@ -62,7 +62,13 @@ export class Fullscreen<
     batch(() => {
       if (camera instanceof PerspectiveCamera) {
         const cameraHeight = 2 * Math.tan((Math.PI * camera.fov) / 360) * distanceToCamera!
-        this.pixelSize.value = cameraHeight / this.renderer.getSize(vectorHelper).y
+        let pixelSize = cameraHeight / this.renderer.getSize(vectorHelper).y
+        //if we are in a screen-based xr session, apply the pixel ratio to the pixel size to display the UI in the same size as outside of XR
+        const xrSession = this.renderer.xr.getSession()
+        if (xrSession?.interactionMode === 'screen-space') {
+          pixelSize *= this.renderer.getPixelRatio()
+        }
+        this.pixelSize.value = pixelSize
         this.sizeY.value = cameraHeight
         this.sizeX.value = cameraHeight * camera.aspect
       }