Created attachment 367949 [details]
Use GDK to list monitors and identify the primary one

Overview
========

In a setup with multiple monitors on Wayland, gdk_display_get_primary_monitor()
returns a null pointer and gdk_monitor_is_primary() returns false for all
monitors, in spite of a primary monitor being configured via the Displays panel
of gnome-control-center. This makes it impossible to determine the primary
monitor via these means.

Steps to Reproduce
==================

1) Connect two monitors to your machine (in my case, it's an ADI monitor
connected via VGA and a DELL via HDMI).

2) Start a GNOME session with Wayland.

3) Configure either of the monitors as primary using Displays.

4) Compile and run the attached program.

Actual Results
==============

Irrespective of which monitor is configured as primary, the output always is:

According to gdk_display_get_primary_monitor(), the primary monitor is (null)
Monitor 0 is DELL U2412M, primary?: no
Monitor 1 is ADI MS A710, primary?: no

Expected Results
================

Say that you've configured the DELL monitor as primary under step 3). The
output should then be:

According to gdk_display_get_primary_monitor(), the primary monitor is DELL U2412M
Monitor 0 is DELL U2412M, primary?: yes
Monitor 1 is ADI MS A710, primary?: no

Build
=====

gtk3 3.22.26+161+g60750b3ffd-1, Arch Linux

Additional Information
======================

Under X11, primary monitor reporting via the means indicated above works fine,
but the ordering of the monitors changes: the primary one is always Monitor 0.
By contrast, under Wayland, the ordering is always the same: DELL first, ADI
second. The ordering in Displays is also always the same but reversed: ADI
first, DELL second.

This suggests that GNOME uses custom code to determine which monitor is which
and which monitor is the primary one. Indeed, it must have a way of identifying
the primary monitor even under Wayland since it puts the top bar on the right
one and Displays shows one of them as primary. However, other GTK applications
are out of luck under Wayland.

Sample outputs under X11
------------------------

Note that under X11, the names reported by gdk_monitor_get_model() are plug
names, VGA-1 and HDMI-1.

With DELL (HDMI-1) set as the primary monitor via Displays:

According to gdk_display_get_primary_monitor(), the primary monitor is HDMI-1
Monitor 0 is HDMI-1, primary?: yes
Monitor 1 is VGA-1, primary?: no

With ADI (VGA-1) set as the primary monitor via Displays:

According to gdk_display_get_primary_monitor(), the primary monitor is VGA-1
Monitor 0 is VGA-1, primary?: yes
Monitor 1 is HDMI-1, primary?: no