Below is an example that inserts a TextField into a parent while the showing.

Run and:
- click add to insert the textField
- expected: textfield inserted and all its content selected
- actual: content not selected

to see that this is a first-time misbehavior only:
- click on remove
- click on add
- expected and actual: all content selected

The culprit: TextFieldBehavior

in its constructor, it installs a listener to scene.focusOwner which handles the selectAll on change but does not handle the intial state:

        textField.sceneProperty().addListener(weakSceneListener);
        if (textField.getScene() != null) {
            textField.getScene().focusOwnerProperty().addListener(weakFocusOwnerListener);
            // FIXME bug or feature: initial selection?
            if (textField.getScene().getFocusOwner() == textField) {
            // missing line
// textField.selectRange(textField.getLength(), 0);
            }
        }

also might be that the re-wiring code (when switching the scene) should update the selection


The example:

public class TextFieldInitialSelection extends Application {

    private Parent createContent() {
        TextField textField = new TextField("just some text");
        Button add = new Button("insert");
        Button remove = new Button("remove");
        VBox content = new VBox(10, add, remove);
        
        add.setOnAction(e -> {
            content.getChildren().add(textField);
            textField.requestFocus();
        });
        remove.setOnAction(e -> {
            content.getChildren().remove(textField);
        });
        return content;
    }

    @Override
    public void start(Stage stage) throws Exception {
        stage.setScene(new Scene(createContent(), 300, 300));
        stage.show();
    }

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