meta-monitor-manager-xrandr.c: fix crtc disabling

CRTCs must also be disabled if the requested screen size is, in any
dimension, smaller than what the screen size would be without any xrandr
scaling applied.  See, e.g., [1, 2].

[1] https://lists.x.org/archives/xorg-devel/2018-March/056203.html
[2] https://github.com/linuxmint/muffin/blob/b15de53d7bc43dbcd0136cd7f845eb7ec9d89e6a/src/backends/x11/meta-monitor-manager-xrandr.c#L851

Author: jknockel

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

@@ -550,7 +550,7 @@ apply_crtc_assignments (MetaMonitorManager *manager,
   MetaX11ScaleMode scale_mode = meta_settings_get_x11_scale_mode (settings);
   unsigned i, valid_crtcs;
   GList *l;
-  int width, height;
+  int width, height, scaled_width, scaled_height;
   float max_scale;
   float avg_screen_scale;
   gboolean have_scaling;
@@ -563,6 +563,7 @@ apply_crtc_assignments (MetaMonitorManager *manager,
   /* Compute the new size of the screen (framebuffer) */
   max_scale = get_maximum_crtc_info_scale (crtcs, n_crtcs);
   width = 0; height = 0;
+  scaled_width = 0; scaled_height = 0;
   avg_screen_scale = 0;
   valid_crtcs = 0;
   for (i = 0; i < n_crtcs; i++)
@@ -578,6 +579,13 @@ apply_crtc_assignments (MetaMonitorManager *manager,
 
       if (have_scaling && scale_mode == META_X11_SCALE_MODE_UI_DOWN)
         scale = (ceilf (max_scale) / crtc_info->scale) * crtc_info->scale;
+      else
+        {
+          scaled_width = MAX (scaled_width, crtc_info->layout.origin.x +
+                              crtc_info->layout.size.width * crtc_info->scale);
+          scaled_height = MAX (scaled_height, crtc_info->layout.origin.y +
+                               crtc_info->layout.size.height * crtc_info->scale);
+        }
 
       width = MAX (width, (int) roundf (crtc_info->layout.origin.x +
                                         crtc_info->layout.size.width * scale));
@@ -609,7 +617,7 @@ apply_crtc_assignments (MetaMonitorManager *manager,
       y2 = (int) roundf (crtc_config->layout.origin.y +
                          crtc_config->layout.size.height);
 
-      if (!crtc_info->mode || x2 > width || y2 > height)
+      if (!crtc_info->mode || width < scaled_width || height < scaled_height || x2 > width || y2 > height)
         {
           xrandr_set_crtc_config (manager_xrandr,
                                   crtc,