If BusyIndicator is initially invisible and its running property is bound to constant value (`true`), when the visible property is `true`, it will be invisible for specific styling: Fusion and macOS, but there might be more. Works as intended for Basic, Material.
An easy fix is to bind the property running to visible, but this issue still feels like a bug since there are mismatching behavior in different styling.

import QtQuick.Window
import QtQuick.Controls.Fusion

Window {
  visible: true
  width: 640
  height: 480
  title: qsTr("Fusion and macOS not working")

  MouseArea {
    id: mouse
    anchors.fill: parent
    onClicked: indicator.visible = !indicator.visible
  }

  BusyIndicator {
    id: indicator
    visible: false
    anchors.centerIn: parent
    running: true //setting visible fixes
  }
}