[mutter] window-xwayland: Add Xwayland fullscreen games workaround

[Carlos Garnacho <carlosg src gnome org>]

commit dbb95cbe5fc13eb323c3b58a8a1d17eba4a2de60
Author: Hans de Goede <hdegoede redhat com>
Date:   Tue Sep 3 16:18:00 2019 +0200

    window-xwayland: Add Xwayland fullscreen games workaround
    
    This is a workaround for X11 games which use randr to change the resolution
    in combination with NET_WM_STATE_FULLSCREEN when going fullscreen.
    
    Newer versions of Xwayland support the randr part of this by supporting randr
    resolution change emulation in combination with using WPviewport to scale the
    app's window (at the emulated resolution) to fill the entire monitor.
    
    Apps using randr in combination with NET_WM_STATE_FULLSCREEN expect the
    fullscreen window to have the size of the emulated randr resolution since
    when running on regular Xorg the resolution will actually be changed and
    after that going fullscreen through NET_WM_STATE_FULLSCREEN will size
    the window to be equal to the new resolution.
    
    We need to emulate this behavior for these games to work correctly.
    
    Xwayland's emulated resolution is a per X11 client setting and Xwayland
    will set a special _XWAYLAND_RANDR_EMU_MONITOR_RECTS property on the
    toplevel windows of a client (and only those of that client), which has
    changed the (emulated) resolution through a randr call.
    
    This commit checks for that property and if it is set adjusts the fullscreen
    monitor rect for this window to match the emulated resolution.
    
    Here is a step-by-step of such an app going fullscreen:
    1. App changes monitor resolution with randr.
    2. Xwayland sets the _XWAYLAND_RANDR_EMU_MONITOR_RECTS property on all the
       apps current and future windows. This property contains the origin of the
       monitor for which the emulated resolution is set and the emulated
       resolution.
    3. App sets _NET_WM_FULLSCREEN.
    4. We check the property and adjust the app's fullscreen size to match
       the emulated resolution.
    5. Xwayland sees a Window at monitor origin fully covering the emulated
       monitor resolution. Xwayland sets a viewport making the emulated
       resolution sized window cover the full actual monitor resolution.
    
    https://gitlab.gnome.org/GNOME/mutter/merge_requests/739

 src/wayland/meta-window-xwayland.c | 89 ++++++++++++++++++++++++++++++++++++++
 src/x11/atomnames.h                |  1 +
 2 files changed, 90 insertions(+)
---
diff --git a/src/wayland/meta-window-xwayland.c b/src/wayland/meta-window-xwayland.c
index 6f073893a..9967f439a 100644
--- a/src/wayland/meta-window-xwayland.c
+++ b/src/wayland/meta-window-xwayland.c
@@ -20,6 +20,7 @@
 
 #include "x11/window-x11.h"
 #include "x11/window-x11-private.h"
+#include "x11/xprops.h"
 #include "wayland/meta-window-xwayland.h"
 #include "wayland/meta-wayland.h"
 
@@ -53,6 +54,93 @@ meta_window_xwayland_init (MetaWindowXwayland *window_xwayland)
 {
 }
 
+/**
+ * meta_window_xwayland_adjust_fullscreen_monitor_rect:
+ *
+ * This function implements a workaround for X11 apps which use randr to change the
+ * the monitor resolution, followed by setting _NET_WM_FULLSCREEN to make the
+ * window-manager fullscreen them.
+ *
+ * Newer versions of Xwayland support the randr part of this by supporting randr
+ * resolution change emulation in combination with using WPviewport to scale the
+ * app's window (at the emulated resolution) to fill the entire monitor.
+ *
+ * Apps using randr in combination with NET_WM_STATE_FULLSCREEN expect the
+ * fullscreen window to have the size of the emulated randr resolution since
+ * when running on regular Xorg the resolution will actually be changed and
+ * after that going fullscreen through NET_WM_STATE_FULLSCREEN will size
+ * the window to be equal to the new resolution.
+ *
+ * We need to emulate this behavior for these apps to work correctly.
+ *
+ * Xwayland's emulated resolution is a per X11 client setting and Xwayland
+ * will set a special _XWAYLAND_RANDR_EMU_MONITOR_RECTS property on the
+ * toplevel windows of a client (and only those of that client), which has
+ * changed the (emulated) resolution through a randr call.
+ *
+ * Here we check for that property and if it is set we adjust the fullscreen
+ * monitor rect for this window to match the emulated resolution.
+ *
+ * Here is a step-by-step of such an app going fullscreen:
+ * 1. App changes monitor resolution with randr.
+ * 2. Xwayland sets the _XWAYLAND_RANDR_EMU_MONITOR_RECTS property on all the
+ *    apps current and future windows. This property contains the origin of the
+ *    monitor for which the emulated resolution is set and the emulated
+ *    resolution.
+ * 3. App sets _NET_WM_FULLSCREEN.
+ * 4. We check the property and adjust the app's fullscreen size to match
+ *    the emulated resolution.
+ * 5. Xwayland sees a Window at monitor origin fully covering the emulated
+ *    monitor resolution. Xwayland sets a viewport making the emulated
+ *    resolution sized window cover the full actual monitor resolution.
+ */
+static void
+meta_window_xwayland_adjust_fullscreen_monitor_rect (MetaWindow    *window,
+                                                     MetaRectangle *fs_monitor_rect)
+{
+  MetaX11Display *x11_display = window->display->x11_display;
+  MetaRectangle win_monitor_rect;
+  cairo_rectangle_int_t *rects;
+  uint32_t *list = NULL;
+  int i, n_items = 0;
+
+  if (!window->monitor)
+    {
+      g_warning ("MetaWindow does not have a monitor");
+      return;
+    }
+
+  win_monitor_rect = meta_logical_monitor_get_layout (window->monitor);
+
+  if (!meta_prop_get_cardinal_list (x11_display,
+                                    window->xwindow,
+                                    x11_display->atom__XWAYLAND_RANDR_EMU_MONITOR_RECTS,
+                                    &list, &n_items))
+    return;
+
+  if (n_items % 4)
+    {
+      meta_verbose ("_XWAYLAND_RANDR_EMU_MONITOR_RECTS on %s has %d values which is not a multiple of 4",
+                    window->desc, n_items);
+      g_free (list);
+      return;
+    }
+
+  rects = (cairo_rectangle_int_t *) list;
+  n_items = n_items / 4;
+  for (i = 0; i < n_items; i++)
+    {
+      if (rects[i].x == win_monitor_rect.x && rects[i].y == win_monitor_rect.y)
+        {
+          fs_monitor_rect->width = rects[i].width;
+          fs_monitor_rect->height = rects[i].height;
+          break;
+        }
+    }
+
+  g_free (list);
+}
+
 static void
 meta_window_xwayland_force_restore_shortcuts (MetaWindow         *window,
                                               ClutterInputDevice *source)
@@ -115,6 +203,7 @@ meta_window_xwayland_class_init (MetaWindowXwaylandClass *klass)
   MetaWindowClass *window_class = META_WINDOW_CLASS (klass);
   GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
 
+  window_class->adjust_fullscreen_monitor_rect = meta_window_xwayland_adjust_fullscreen_monitor_rect;
   window_class->force_restore_shortcuts = meta_window_xwayland_force_restore_shortcuts;
   window_class->shortcuts_inhibited = meta_window_xwayland_shortcuts_inhibited;
 
diff --git a/src/x11/atomnames.h b/src/x11/atomnames.h
index b61f47ff4..4099d5d31 100644
--- a/src/x11/atomnames.h
+++ b/src/x11/atomnames.h
@@ -81,6 +81,7 @@ item(ATOM_PAIR)
 item(_XKB_RULES_NAMES)
 item(WL_SURFACE_ID)
 item(_XWAYLAND_MAY_GRAB_KEYBOARD)
+item(_XWAYLAND_RANDR_EMU_MONITOR_RECTS)
 
 /* Oddities: These are used, and we need atoms for them,
  * but when we need all _NET_WM hints (i.e. when we're making