FULL PRODUCT VERSION :
java version "1.8.0_92"
Java(TM) SE Runtime Environment (build 1.8.0_92-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.92-b14, mixed mode)


ADDITIONAL OS VERSION INFORMATION :
OSX 11, Ubuntu 14, Ubuntu 16

A DESCRIPTION OF THE PROBLEM :
First show() on ContextMenu seems to not fully apply the css ofthe anchor Node. This results in a gap between the anchor and context menu(see source code example)

When show is called the a second time the problem is gone.

As you can see in source code this happens if i have a parent css with a non standard font size set.

Tested on u66, u74 and u92

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Start test case application
Push button

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
ContextMenu show be shown just above the button
ACTUAL -
ContextMenu is shown above the button with a gap
The second time you press the button the context menu is shown just above the button(correct behavior)

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
package contextmeu.bug.example

import javafx.application.Application;
import javafx.geometry.Side;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.ContextMenu;
import javafx.scene.control.MenuItem;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

public class main extends Application {

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

   @Override
   public void start(Stage primaryStage) {
       ContextMenu menu = new ContextMenu();
       menu.getItems().add(new MenuItem("123"));

       Button button = new Button();
       button.setStyle("-fx-font-size: 0.50em;");
       button.setOnAction(event -> menu.show(button, Side.TOP, 0, 0));

       Scene scene = new Scene(new StackPane(button), 800, 800);
       primaryStage.setScene(scene);
       primaryStage.show();
   }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Calling show() on the menu twice.