Run this sample test, you don't need an actual "test.css" in the project.

"import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.ContextMenu;
import javafx.scene.control.MenuItem;
import javafx.scene.control.Tab;
import javafx.scene.control.TabBuilder;
import javafx.scene.control.TabPane;
import javafx.scene.control.TabPaneBuilder;
import javafx.stage.Stage;

public class TestTabPane extends Application {

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

    @Override
    public void start(Stage paramStage) throws Exception {
        TabPane tabPane = TabPaneBuilder.create().build();
        for (int i = 1; i <= 10; i++) {
            tabPane.getTabs().add(buildTab("tab" + i));
        }
        Scene scene = new Scene(tabPane, 300, 275);
        scene.getStylesheets().add("test.css");
        paramStage.setScene(scene);
        paramStage.show();
    }

    public Tab buildTab(String name) {
        final Tab tab = TabBuilder.create().closable(true).build();
        tab.setText(name);
        ContextMenu contextMenu = new ContextMenu();
        contextMenu.getItems().add(new MenuItem("test"));
        tab.setContextMenu(contextMenu);
        return tab;
    }
}
"

Run the sample
Right-click on tab1 header, contextMenu is displayed properly.
Right-click again on tab1 header (while the contextMenu is still displayed), the first contextMenu is hidden, a new ContextMenu is shown, but that time, the right and bottom edge are cut.

I really don't know why it is happening. Only thing I know is that adding a Stylesheet (present in the project or not) in the Scene cause that issue.

Tested with JDK8u112.