Renders a dashed crosshair on JavaFX canvas when mouse is dragged over it

crosshair.kt

fun GraphicsContext.group(op: GraphicsContext.() -> Unit) {
    save()
    op(this)
    restore()
}

// ...

val ctx = canvas.graphicsContext2D
canvas.onMouseDragged = EventHandler { e ->
    ctx.group {
        setLineDashes(4.0, 4.0)
        stroke = Color.RED
        val offset = -.5 * lineWidth // for nice pixel grid alignment
        
        beginPath()
        moveTo(e.x + offset, 0.0)
        lineTo(e.x + offset, ctx.canvas.height)

        moveTo(0.0, e.y + offset)
        lineTo(ctx.canvas.width, e.y + offset)
        stroke()
    }
}