[mutter] monitor-manager: Find active monitor when deriving global scale

[Marco Trevisan <marcotrevi src gnome org>]

commit 0b3a1c9c314a88c57668437245d54b200537e619
Author: Jonas Ådahl <jadahl gmail com>
Date:   Wed Apr 25 10:43:11 2018 +0200

    monitor-manager: Find active monitor when deriving global scale
    
    When deriving the global scale directly from the current hardware state
    (as done when using the X11 backend) we are inspecting the logical
    state they had prior to the most recent hot plug. That means that a
    primary monitor might have been disabled, and a new primary monitor may
    not have been assigned yet.
    
    Stop assuming a primary monitor has an active mode before having
    reconstructed the logical state by finding some active monitor if the
    old primary monitor was disabled. This avoids a crash when trying to
    derive the global scale from a disabled monitor.
    
    Closes: https://gitlab.gnome.org/GNOME/mutter/issues/130

 src/backends/meta-monitor-manager.c | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)
---
diff --git a/src/backends/meta-monitor-manager.c b/src/backends/meta-monitor-manager.c
index 829c189ea..0b5e54c48 100644
--- a/src/backends/meta-monitor-manager.c
+++ b/src/backends/meta-monitor-manager.c
@@ -89,6 +89,9 @@ static gboolean
 meta_monitor_manager_is_config_complete (MetaMonitorManager *manager,
                                          MetaMonitorsConfig *config);
 
+static MetaMonitor *
+meta_monitor_manager_get_active_monitor (MetaMonitorManager *manager);
+
 MetaBackend *
 meta_monitor_manager_get_backend (MetaMonitorManager *manager)
 {
@@ -198,13 +201,17 @@ calculate_monitor_scale (MetaMonitorManager *manager,
 static float
 derive_calculated_global_scale (MetaMonitorManager *manager)
 {
-  MetaMonitor *primary_monitor;
+  MetaMonitor *monitor = NULL;
+
+  monitor = meta_monitor_manager_get_primary_monitor (manager);
 
-  primary_monitor = meta_monitor_manager_get_primary_monitor (manager);
-  if (!primary_monitor)
+  if (!monitor || !meta_monitor_is_active (monitor))
+    monitor = meta_monitor_manager_get_active_monitor (manager);
+
+  if (!monitor)
     return 1.0;
 
-  return calculate_monitor_scale (manager, primary_monitor);
+  return calculate_monitor_scale (manager, monitor);
 }
 
 static float
@@ -2378,6 +2385,12 @@ meta_monitor_manager_get_laptop_panel (MetaMonitorManager *manager)
   return find_monitor (manager, meta_monitor_is_laptop_panel);
 }
 
+static MetaMonitor *
+meta_monitor_manager_get_active_monitor (MetaMonitorManager *manager)
+{
+  return find_monitor (manager, meta_monitor_is_active);
+}
+
 MetaMonitor *
 meta_monitor_manager_get_monitor_from_connector (MetaMonitorManager *manager,
                                                  const char         *connector)