Drop down menus appear in wrong position when using multiple monitors arranged vertically

[Paul Crockett]

I am using multiple monitors arranged as shown below:

+------------++------------+
|______3_____||______2_____|
|____________||____________|
+------------++------------+
   +---------+
   |____1____|
   |_________|
   +---------+

Monitors 1 and 3 have different resolutions (1920x1080 and 1600x900) and are arranged so that their right edges align.

When opening a drop down menu (such as the 'File' menu in terminal) that appears to the top left of a full-screen application on monitor 3, the drop down menu position is offset horizontally. It is as if menus that appear at the left of monitor 3 are 'pushed' to the right so that they are in line with monitor 1.

The attached screen capture shows roughly where the File menu drop down appears.

Menus on monitors 1 and 2 appear in the expected location.

I imagine this is a problem with GNOME 3, as I never had this problem with Unity on Ubuntu 16, and it seems to affect all applications.

Description: Ubuntu 18.04.1 LTS
Release: 18.04

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: gnome-terminal 3.28.2-1ubuntu1~18.04.1
ProcVersionSignature: Ubuntu 4.15.0-38.41-generic 4.15.18
Uname: Linux 4.15.0-38-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.4
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Thu Nov 8 15:26:34 2018
ExecutablePath: /usr/lib/gnome-terminal/gnome-terminal-server
InstallationDate: Installed on 2018-11-02 (6 days ago)
InstallationMedia: Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
ProcEnviron:
 LANG=en_GB.UTF-8
 LANGUAGE=en_GB:en
 PATH=(custom, no user)
 SHELL=/bin/bash
 XDG_RUNTIME_DIR=<set>
SourcePackage: gnome-terminal
UpgradeStatus: No upgrade log present (probably fresh install)