FULL PRODUCT VERSION :
Java(TM) SE Runtime Environment (build 1.8.0_144-b01)
Java HotSpot(TM) 64-Bit Server VM (build 25.144-b01, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 6.1.7601]

A DESCRIPTION OF THE PROBLEM :
Application started with JavaWebStart displays JMenu popup on wrong screen, if you extend your desktop by a second screen without restarting the JavaWebStart application.

I have also tested this with the latest early access build jdk-9+181 and the problem still exists.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
You need two screens

1.Disconnect second screen, so that you have only one active ( "Windows + p" for windows disconnect)
2.Open web start from : https://docs.oracle.com/javase/tutorialJWS/samples/uiswing/ActionDemoProject/ActionDemo.jnlp
3.Click "Menu" all fine
4.Connect second screen ( "Windows + p" for windows) extended screen
5.Click "Menu"  all fine
6.Move application window to second screen (on the screen which was active in step 2, where you started the application)
7.Click "Menu" => Items appear on wrong screen 

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
I expected the same behaviour as in step 3. The MenuItems in step 7 should appear under the JMenu on the same screen as the application.
ACTUAL -
The application is displayed on one screen and the MenuItems popup appears on the other screen.

REPRODUCIBILITY :
This bug can be reproduced always.

CUSTOMER SUBMITTED WORKAROUND :
The only solution I have found is to restart your application if the second screen is connected.

This is a big problem for many of our customers. Because they use their laptops over the day in different environments with different screens and always have to restart the application to solve the problem.