Bug 1621781 - Have APZ ignore mouse events when RDM is active. r=botond

nsEventStatus AsyncPanZoomController::HandleDragEvent(

    const MouseInput& aEvent, const AsyncDragMetrics& aDragMetrics,

    CSSCoord aInitialThumbPos) {

  if (!StaticPrefs::apz_drag_enabled()) {

  // RDM is a special case where touch events will be synthesized in response

  // to mouse events, and APZ will receive both even though RDM prevent-defaults

  // the mouse events. This is because mouse events don't opt into APZ waiting

  // to check if the event has been prevent-defaulted and are still processed

  // as a result. To handle this, have APZ ignore mouse events when RDM is

  // active.

  if (!StaticPrefs::apz_drag_enabled() ||

      mScrollMetadata.GetIsRDMTouchSimulationActive()) {

    return nsEventStatus_eIgnore;

  }