SGraphEditor::OnFocused called only when clicked the middle mouse button

Development Pipeline & Plugins
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UE4 version:4.22.3

I am making a plugin based on the SGraphEditor.I need to do something when user focused on the graph.I create the SGraphEditor like this

SGraphEditor::FGraphEditorEvents InEvents;
	InEvents.OnSelectionChanged = SGraphEditor::FOnSelectionChanged::CreateSP(this,&FTestGraphAssetEditor::OnSelectedNodesChanged);
	InEvents.OnNodeDoubleClicked = FSingleNodeEvent::CreateSP(this,&FTestGraphAssetEditor::OnNodeDoubleClicked);
	InEvents.OnTextCommitted = FOnNodeTextCommitted::CreateSP(this,&FTestGraphAssetEditor::OnNodeTitleCommitted);
	InEvents.OnFocused = SGraphEditor::FOnFocused::CreateSP(this,&FTestGraphAssetEditor::OnFocused);
	return SNew(SGraphEditor)
		.AdditionalCommands(GraphEditorCommands)
		.IsEditable(true)
		.Appearance(AppearanceInfo)
		.GraphToEdit(EditingGraph->EdGraph)
		.GraphEvents(InEvents)
		.AutoExpandActionMenu(true)
		.ShowGraphStateOverlay(false);

The OnFocused fuction will called only when I clicked middle mouse button on the graph,but the root class function’SWidget::OnFocusReceived(const FGeometry& MyGeometry, const FFocusEvent& InFocusEvent)’ will called as soon as focued on the graph.
I think they are the same action,but there are actually called in different different way.