1. As described in title, pressing Alt+F4 on floating mode hiding element even if CanHide is false.
   AvalonDock: Can hide LayoutAnchorable using Alt+F4 when CanHide is false xceedsoftware/wpftoolkit#1516

2. Avoid NullReferenceException() in DockingManager.GetFloatingWindowsByZOrder() by checking ctrl.Model.Root != null before accessing Manager:

      if( ctrl != null && ctrl.Model.Root != null && ctrl.Model.Root.Manager == this )
        yield return ctrl;

3. Test Case
   • Create 2 documents via File > New
   • drag one document into a FloatingWindow position and select Close All But this from drop down menu of FloatingDocumentWindow
   • Bug: System.ArgumentOutOfRangeException: 'Index must be within the bounds of the List.
     in DocumentPaneDropTargets
     in case DropTargetType.DocumentPaneDockInside:
     At about Line 225 ...paneModel.Children.Insert( i, sourceModel );
      
     Fixed with:
     paneModel.Children.Insert( Math.Min(paneModel.Children.Count, i), sourceModel );

Review Fix in this branch
https://github.com/Dirkster99/AvalonDock/tree/FloatingWindowCanHideFix