Final Code for BlurSurface

BlurSurface.kt

@Composable
fun BlurSurface(
    modifier: Modifier,
    parent: ViewGroup
) {
    Surface(
        modifier = modifier,
        color = Color.Transparent
    ) {
        AndroidView(
            factory = {
                BlurSurfaceView(parent.context)
            },
            modifier = Modifier
                .fillMaxSize(),
            update = { blurView ->
                blurView.setParent(
                    parent = parent
                )
            }
        )
    }
}

BlurSurfaceView_Final.kt

class BlurSurfaceView: View {
    private var canvas: Canvas? = null
    private lateinit var bitmap: Bitmap
    private lateinit var parent: ViewGroup

    private var renderScript: RenderScript? = null
    private lateinit var blurScript: ScriptIntrinsicBlur
    private lateinit var outAllocation: Allocation

    constructor(context: Context) : super(context)

    constructor(context: Context, attrs: AttributeSet?) : super(context, attrs)

    override fun onSizeChanged(w: Int, h: Int, oldw: Int, oldh: Int) {
        super.onSizeChanged(w, h, oldw, oldh)
        init(measuredWidth, measuredHeight)
    }

    private fun init(measuredWidth: Int, measuredHeight: Int) {
        bitmap = Bitmap.createBitmap(
            measuredWidth,
            measuredHeight,
            Bitmap.Config.ARGB_8888
        )
        canvas = Canvas(bitmap)
    }

    override fun onDraw(canvas: Canvas) {
        super.onDraw(canvas)

        canvas.save()
        canvas.drawBitmap(bitmap, 0f, 0f, null)
        canvas.restore()
    }

    private fun getBackgroundAndDrawBehind() {
        //Arrays to store the co-ordinates
        val rootLocation = IntArray(2)
        val viewLocation = IntArray(2)

        parent.getLocationOnScreen(rootLocation) //get the parent co-ordinates
        this.getLocationOnScreen(viewLocation) //get view co-ordinates

        //Calculate relative co-ordinates
        val left: Int = viewLocation[0] - rootLocation[0]
        val top: Int = viewLocation[1] - rootLocation[1]

        canvas?.save()
        canvas?.translate(-left.toFloat(), -top.toFloat()) //translates the initial position
        canvas?.let {
            parent.draw(it)
        }
        canvas?.restore()
        blurWithRenderScript()
    }

    private fun blurWithRenderScript() {
        renderScript = RenderScript.create(context)
        blurScript = ScriptIntrinsicBlur.create(renderScript, Element.U8_4(renderScript))

        val inAllocation = Allocation.createFromBitmap(renderScript, bitmap)
        outAllocation = Allocation.createTyped(renderScript, inAllocation.type)
        blurScript.setRadius(20f)
        blurScript.setInput(inAllocation)

        blurScript.forEach(outAllocation)
        outAllocation.copyTo(bitmap)

        inAllocation.destroy()
    }

    fun setParent(parent: ViewGroup){
        this.parent = parent
        this.parent.viewTreeObserver.removeOnPreDrawListener(drawListener)
        this.parent.viewTreeObserver.addOnPreDrawListener(drawListener)
    }

    private val drawListener =
        ViewTreeObserver.OnPreDrawListener {
            getBackgroundAndDrawBehind()
            true
        }
}

MainActivity_Final.kt

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        val decorView = window.decorView
        val rootView = decorView.findViewById(android.R.id.content) as ViewGroup

        setContent {
            Theme {
                Surface(
                    modifier = Modifier.fillMaxSize(),
                    color = Color.Black
                ) {
                    val context = LocalContext.current

                    Box(
                        modifier = Modifier
                            .fillMaxSize()
                            .horizontalScroll(rememberScrollState())
                    ) {
                        Image(
                            painter = painterResource(id = R.drawable.background),
                            contentDescription = "background",
                            contentScale = ContentScale.FillHeight
                        )
                    }

                    Box(
                        modifier = Modifier
                            .wrapContentSize()
                            .border(
                                width = 2.dp,
                                color = Color.Yellow
                            )
                    ) {
                        BlurSurface(
                            modifier = Modifier.size(240.dp),
                            parent = rootView
                        )
                    }

                }
            }
        }
    }
}