To reproduce, run the example below
- for visualization header/fillter/corner background is blue/yellow/red
- uncheck show-menubutton
- expected: filler (== yellow) stretches to trailing corner
- actual: corner region is blue, that is the header's background
- only after resizing a column, the filler resizes to fill the corner region

this happens only on the very first appearance with menuButton visible

example code:

import java.net.URL;
import java.util.Locale;
import javafx.application.Application;
import javafx.collections.FXCollections;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.CheckBox;
import javafx.scene.control.TableColumn;
import javafx.scene.control.TableView;
import javafx.scene.control.cell.PropertyValueFactory;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.HBox;
import javafx.stage.Stage;

public class TableHeaderMenuButtonBug extends Application {

    private Parent getContent() {
        TableView<Locale> table = new TableView<>(FXCollections.observableArrayList(
                Locale.getAvailableLocales()));
        TableColumn<Locale, String> countryCode = new TableColumn<>("CountryCode");
        countryCode.setCellValueFactory(new PropertyValueFactory<>("country"));
        table.getColumns().addAll(countryCode);
        
        table.setTableMenuButtonVisible(true);
        CheckBox menuButtonVisible = new CheckBox("Show MenuButton");
        menuButtonVisible.selectedProperty().bindBidirectional(table.tableMenuButtonVisibleProperty());
        HBox buttons = new HBox(10, menuButtonVisible);
        BorderPane pane = new BorderPane(table);
        pane.setBottom(buttons);
        return pane;
    }

    @Override
    public void start(Stage primaryStage) throws Exception {
        primaryStage.setScene(new Scene(getContent(), 800, 400));
        URL uri = getClass().getResource("headers.css");
        primaryStage.getScene().getStylesheets().add(uri.toExternalForm());
        primaryStage.show();

    }
    
    public static void main(String[] args) {
        launch(args);
    }

}

headers.css:

.column-header-background {
-fx-background-color: blue;

}
.column-header-background > .show-hide-columns-button {
-fx-background-color: red;

}
.column-header-background > .filler {
-fx-background-color: yellow;
}