monitor-manager/xrandr: Use a single supported scales list for all

Under X11 we can only ever have the same scale configured on all
monitors. In order to use e.g. scale 2 when there is a HiDPI monitor
connected, we must not disallow it because there is a monitor that does
not support scale 2. Thus we must show the same scale for every monitor
and monitor mode, even though it might result in a bad experience.

Do this by iterating through all the monitors adding all supported
scales by the preferred mode, combining all the supported scales. This
supported scales list is then used for all monitor and modes no matter
what.

https://bugzilla.gnome.org/show_bug.cgi?id=788901

src/backends/x11/meta-monitor-manager-xrandr.c

@@ -74,6 +74,9 @@ struct _MetaMonitorManagerXrandr
 #ifdef HAVE_XRANDR15
   GHashTable *tiled_monitor_atoms;
 #endif /* HAVE_XRANDR15 */
+
+  float *supported_scales;
+  int n_supported_scales;
 };
 
 struct _MetaMonitorManagerXrandrClass
@@ -546,6 +549,17 @@ meta_monitor_manager_xrandr_ensure_initial_config (MetaMonitorManager *manager)
   meta_monitor_manager_update_logical_state_derived (manager, config);
 }
 
+static void
+meta_monitor_manager_xrandr_rebuild_derived (MetaMonitorManager *manager,
+                                             MetaMonitorsConfig *config)
+{
+  MetaMonitorManagerXrandr *manager_xrandr =
+    META_MONITOR_MANAGER_XRANDR (manager);
+
+  g_clear_pointer (&manager_xrandr->supported_scales, g_free);
+  meta_monitor_manager_rebuild_derived (manager, config);
+}
+
 static gboolean
 meta_monitor_manager_xrandr_apply_monitors_config (MetaMonitorManager      *manager,
                                                    MetaMonitorsConfig      *config,
@@ -557,7 +571,7 @@ meta_monitor_manager_xrandr_apply_monitors_config (MetaMonitorManager      *mana
 
   if (!config)
     {
-      meta_monitor_manager_rebuild_derived (manager, NULL);
+      meta_monitor_manager_xrandr_rebuild_derived (manager, NULL);
       return TRUE;
     }
 
@@ -591,7 +605,7 @@ meta_monitor_manager_xrandr_apply_monitors_config (MetaMonitorManager      *mana
         }
       else
         {
-          meta_monitor_manager_rebuild_derived (manager, config);
+          meta_monitor_manager_xrandr_rebuild_derived (manager, config);
         }
     }
 
@@ -832,6 +846,76 @@ meta_monitor_manager_xrandr_calculate_monitor_mode_scale (MetaMonitorManager *ma
   return meta_monitor_calculate_mode_scale (monitor, monitor_mode);
 }
 
+static void
+add_supported_scale (GArray *supported_scales,
+                     float   scale)
+{
+  unsigned int i;
+
+  for (i = 0; i < supported_scales->len; i++)
+    {
+      float supported_scale = g_array_index (supported_scales, float, i);
+
+      if (scale == supported_scale)
+        return;
+    }
+
+  g_array_append_val (supported_scales, scale);
+}
+
+static int
+compare_scales (gconstpointer a,
+                gconstpointer b)
+{
+  float f = *(float *) a - *(float *) b;
+
+  if (f < 0)
+    return -1;
+  if (f > 0)
+    return 1;
+  return 0;
+}
+
+static void
+ensure_supported_monitor_scales (MetaMonitorManager *manager)
+{
+  MetaMonitorManagerXrandr *manager_xrandr =
+    META_MONITOR_MANAGER_XRANDR (manager);
+  MetaMonitorScalesConstraint constraints;
+  GList *l;
+  GArray *supported_scales;
+
+  if (manager_xrandr->supported_scales)
+    return;
+
+  constraints = META_MONITOR_SCALES_CONSTRAINT_NO_FRAC;
+  supported_scales = g_array_new (FALSE, FALSE, sizeof (float));
+
+  for (l = manager->monitors; l; l = l->next)
+    {
+      MetaMonitor *monitor = l->data;
+      MetaMonitorMode *monitor_mode;
+      float *monitor_scales;
+      int n_monitor_scales;
+      int i;
+
+      monitor_mode = meta_monitor_get_preferred_mode (monitor);
+      monitor_scales =
+        meta_monitor_calculate_supported_scales (monitor,
+                                                 monitor_mode,
+                                                 constraints,
+                                                 &n_monitor_scales);
+
+      for (i = 0; i < n_monitor_scales; i++)
+        add_supported_scale (supported_scales, monitor_scales[i]);
+      g_array_sort (supported_scales, compare_scales);
+    }
+
+  manager_xrandr->supported_scales = (float *) supported_scales->data;
+  manager_xrandr->n_supported_scales = supported_scales->len;
+  g_array_free (supported_scales, FALSE);
+}
+
 static float *
 meta_monitor_manager_xrandr_calculate_supported_scales (MetaMonitorManager          *manager,
                                                         MetaLogicalMonitorLayoutMode layout_mode,
@@ -839,12 +923,14 @@ meta_monitor_manager_xrandr_calculate_supported_scales (MetaMonitorManager
                                                         MetaMonitorMode             *monitor_mode,
                                                         int                         *n_supported_scales)
 {
-  MetaMonitorScalesConstraint constraints;
+  MetaMonitorManagerXrandr *manager_xrandr =
+    META_MONITOR_MANAGER_XRANDR (manager);
 
-  constraints = META_MONITOR_SCALES_CONSTRAINT_NO_FRAC;
+  ensure_supported_monitor_scales (manager);
-  return meta_monitor_calculate_supported_scales (monitor, monitor_mode,
+
-                                                  constraints,
+  *n_supported_scales = manager_xrandr->n_supported_scales;
-                                                  n_supported_scales);
+  return g_memdup (manager_xrandr->supported_scales,
+                   manager_xrandr->n_supported_scales * sizeof (float));
 }
 
 static MetaMonitorManagerCapability
@@ -929,8 +1015,8 @@ meta_monitor_manager_xrandr_finalize (GObject *object)
   MetaMonitorManagerXrandr *manager_xrandr = META_MONITOR_MANAGER_XRANDR (object);
 
   g_clear_object (&manager_xrandr->gpu);
-
   g_hash_table_destroy (manager_xrandr->tiled_monitor_atoms);
+  g_free (manager_xrandr->supported_scales);
 
   G_OBJECT_CLASS (meta_monitor_manager_xrandr_parent_class)->finalize (object);
 }
@@ -1014,7 +1100,7 @@ meta_monitor_manager_xrandr_handle_xevent (MetaMonitorManagerXrandr *manager_xra
           config = NULL;
         }
 
-      meta_monitor_manager_rebuild_derived (manager, config);
+      meta_monitor_manager_xrandr_rebuild_derived (manager, config);
     }
 
   return TRUE;