ADDITIONAL SYSTEM INFORMATION :
Microsoft Windows [Version 10.0.18363.1198]
openjdk version "11.0.5" 2019-10-15
OpenJDK Runtime Environment AdoptOpenJDK (build 11.0.5+10)
OpenJDK 64-Bit Server VM AdoptOpenJDK (build 11.0.5+10, mixed mode)

A DESCRIPTION OF THE PROBLEM :
Initial column autosizing does not take graphics into computing pref width of column header.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Open provided test case.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Column header text should be fully visible.
ACTUAL -
Column header is shortened on initial table show, double click on column header reorder line makes autosize works good and fit the header graphics.

---------- BEGIN SOURCE ----------
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

import javafx.application.Application;
import javafx.beans.property.SimpleObjectProperty;
import javafx.collections.FXCollections;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.control.TableColumn;
import javafx.scene.control.TableView;
import javafx.scene.control.TreeItem;
import javafx.scene.control.TreeTableColumn;
import javafx.scene.control.TreeTableView;
import javafx.scene.control.cell.MapValueFactory;
import javafx.scene.layout.HBox;
import javafx.stage.Stage;

public class GraphicsInHeaderColumnTest
{

    public static void main( String[] args )
    {
        Application.launch( FxApp.class, args );
    }

    public static class FxApp extends Application
    {

        @Override
        public void start( final Stage primaryStage ) throws Exception
        {
            System.err.println( System.getProperty( "javafx.version" ) );
            final var rootPane = new HBox( 5d );
            final TableView< Map< String, Object > > tableView = createTableView();
            rootPane.getChildren().add( tableView );

            final TreeTableView< Map< String, Object > > treeTableView = createTreeTableView();
            rootPane.getChildren().add( treeTableView );

            primaryStage.setScene( new Scene( rootPane, 800, 600 ) );
            primaryStage.show();
        }

        private TreeTableView< Map< String, Object > > createTreeTableView()
        {
            final TreeTableView< Map< String, Object > > treeTableView = new TreeTableView<>();
            treeTableView.setShowRoot( false );
            final TreeItem< Map< String, Object > > root = new TreeItem<>();
            for( int i = 0; i < 10; i++ )
            {
                final Map< String, Object > data = Map.of( "id", i, "data", "row" + i );
                final TreeItem< Map< String, Object > > treeItem = new TreeItem<>( data );
                root.getChildren().add( treeItem );
            }
            final var idCol = new TreeTableColumn< Map< String, Object >, Object >( "id" );
            idCol.setCellValueFactory( param -> {
                final var value = param.getValue();
                if( value == null || value.getValue() == null )
                {
                    return null;
                }
                return new SimpleObjectProperty<>( value.getValue().get( "id" ) );
            } );
            idCol.setText( null );
            idCol.setGraphic( new Label( "Long text 'id'" ) );

            final var dataCol = new TreeTableColumn< Map< String, Object >, Object >( "data" );
            dataCol.setCellValueFactory( param -> {
                final var value = param.getValue();
                if( value == null || value.getValue() == null )
                {
                    return null;
                }
                return new SimpleObjectProperty<>( value.getValue().get( "data" ) );
            } );
            dataCol.setText( null );
            dataCol.setGraphic( new Label( "Long text 'data'" ) );
            treeTableView.getColumns().addAll( idCol, dataCol );
            treeTableView.setRoot( root );
            return treeTableView;
        }

        private TableView< Map< String, Object > > createTableView()
        {
            final TableView< Map< String, Object > > tableView = new TableView<>( FXCollections
                .observableArrayList( IntStream.range( 0, 10 )
                    .mapToObj( i -> Map.< String, Object >of( "id", i, "data", "row_" + i ) )
                    .collect( Collectors.toList() ) ) );
            final var idCol = new TableColumn< Map, Object >( "id" );
            idCol.setCellValueFactory( new MapValueFactory< Object >( "id" ) );
            idCol.setText( null );
            idCol.setGraphic( new Label( "Long text 'id'" ) );

            final var dataCol = new TableColumn< Map, Object >( "data" );
            dataCol.setCellValueFactory( new MapValueFactory< Object >( "data" ) );
            dataCol.setText( null );
            dataCol.setGraphic( new Label( "Long text 'data'" ) );
            tableView.getColumns().add( (TableColumn)idCol );
            tableView.getColumns().add( (TableColumn)dataCol );
            return tableView;
        }

    }

}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
run autosizing for all columns after it is shown