Here's a sample program which reproduces the problem.
If I run and select the "SplitPane Tab", the tab content is not rendered until I resize the Stage.
Note that this seems to be a regression, as I can reproduce it with Java8 (build 120), but not with Java7.

import javafx.application.Application;
import javafx.collections.FXCollections;
import javafx.collections.ObservableList;
import javafx.scene.Scene;
import javafx.scene.control.ListView;
import javafx.scene.control.SplitPane;
import javafx.scene.control.Tab;
import javafx.scene.control.TabPane;
import javafx.stage.Stage;

public class SplitPaneSample extends Application {
public SplitPaneSample() {
}

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

@Override
public void start(Stage stage) {
SplitPane splitPane = new SplitPane();
splitPane.getItems().addAll(createListView(), createListView());

TabPane tabPane = new TabPane();
Tab splitTab = new Tab("SplitPane Tab");
splitTab.setContent(splitPane);
tabPane.getTabs().addAll(new Tab("Empty Tab"), splitTab);

Scene scene = new Scene(tabPane, 500, 500);
stage.setScene(scene);
stage.setTitle("SplitPane Inside TabPane Sample");
stage.show();
}

private ListView<String> createListView() {
ObservableList<String> items = FXCollections.observableArrayList("Animal", "Mineral", "Vegetable");
ListView<String> listView = new ListView<String>(items);
return listView;
}

}