"New Document" opens a new window

[Wes]

To reproduce:

  - Open file(s) in Gedit
  - Right click dock icon -> "New Document"

Expected: gedit opens a new tab in the existing window for the new document.
This is the default behaviour, observed on Fedora.

Actual:
Ubuntu gedit opens a new window for the new document.
Possibly related to https://askubuntu.com/questions/75671/why-does-gedit-keep-randomly-opening-new-instances-when-opening-files-from-nauti

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: gedit 3.28.1-1ubuntu1
ProcVersionSignature: User Name 4.15.0-39.42-generic 4.15.18
Uname: Linux 4.15.0-39-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.5
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Tue Dec 4 12:55:02 2018
EcryptfsInUse: Yes
InstallationDate: Installed on 2018-09-12 (82 days ago)
InstallationMedia: Ubuntu 16.04.5 LTS "Xenial Xerus" - Release amd64 (20180731)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: gedit
UpgradeStatus: Upgraded to bionic on 2018-09-28 (66 days ago)