Add an "Above_Tab" pseudo-keysym

We want switching between the windows of an application to be an easily
accessible operation. The convenient and memorable keybinding is the
key above the tab key - but the keysym for that key isn't consistent
across different keyboard layouts.

Add code that figures out the key from the XKB geometry and a magic
keysym name "Above_Tab" that refers to this key and switch
the default binding for cycle_group to <Alt>Above_Tab.

https://bugzilla.gnome.org/show_bug.cgi?id=635569

7 files changed, 307 insertions, 5 deletions

diff --git a/src/Makefile.am b/src/Makefile.am
index bd3420f9..ef9a3a07 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -17,6 +17,7 @@ metacity_SOURCES= 				\
 	compositor/compositor-xrender.c		\
 	compositor/compositor-xrender.h		\
 	include/compositor.h			\
+	core/above-tab-keycode.c		\
 	core/constraints.c			\
 	core/constraints.h			\
 	core/core.c				\
diff --git a/src/core/above-tab-keycode.c b/src/core/above-tab-keycode.c
new file mode 100644
index 00000000..56966ab0
--- /dev/null
+++ b/src/core/above-tab-keycode.c
@@ -0,0 +1,241 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*- */
+
+/* Find the keycode for the key above the tab key */
+/*
+ * Copyright 2010 Red Hat, Inc.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
+ * 02111-1307, USA.
+ */
+
+/* The standard cycle-windows keybinding should be the key above the
+ * tab key. This will have a different keysym on different keyboards -
+ * it's the ` (grave) key on US keyboards but something else on many
+ * other national layouts. So we need to figure out the keycode for
+ * this key without reference to key symbol.
+ *
+ * The "correct" way to do this is to get the XKB geometry from the
+ * X server, find the Tab key, find the key above the Tab key in the
+ * same section and use the keycode for that key. This is what I
+ * implemented here, but unfortunately, fetching the geometry is rather
+ * slow (It could take 20ms or more.)
+ *
+ * If you looking for a way to optimize Mutter startup performance:
+ * On all Linux systems using evdev the key above TAB will have
+ * keycode 49. (KEY_GRAVE=41 + the 8 code point offset between
+ * evdev keysyms and X keysyms.) So a configure option
+ * --with-above-tab-keycode=49 could be added that bypassed this
+ * code. It wouldn't work right for displaying Mutter remotely
+ * to a non-Linux X server, but that is pretty rare.
+ */
+
+#include <config.h>
+
+#include <string.h>
+
+#include "display-private.h"
+
+#include <X11/keysym.h>
+
+#ifdef HAVE_XKB
+#include <X11/XKBlib.h>
+#include <X11/extensions/XKBgeom.h>
+
+static guint
+compute_above_tab_keycode (Display *xdisplay)
+{
+  XkbDescPtr keyboard;
+  XkbGeometryPtr geometry;
+  int i, j, k;
+  int tab_keycode;
+  char *tab_name;
+  XkbSectionPtr tab_section;
+  XkbBoundsRec tab_bounds;
+  XkbKeyPtr best_key = NULL;
+  guint best_keycode = (guint)-1;
+  int best_x_dist = G_MAXINT;
+  int best_y_dist = G_MAXINT;
+
+  /* We need only the Names and the Geometry, but asking for these results
+   * in the Keyboard information retrieval failing for unknown reasons.
+   * (Testing with xorg-1.9.1.) So we ask for a part that we don't need
+   * as well.
+   */
+  keyboard = XkbGetKeyboard (xdisplay,
+                             XkbGBN_ClientSymbolsMask | XkbGBN_KeyNamesMask | XkbGBN_GeometryMask,
+                             XkbUseCoreKbd);
+
+  geometry = keyboard->geom;
+
+  /* There could potentially be multiple keys with the Tab keysym on the keyboard;
+   * but XKeysymToKeycode() returns us the one that the alt-Tab binding will
+   * use which is good enough
+   */
+  tab_keycode = XKeysymToKeycode (xdisplay, XK_Tab);
+  if (tab_keycode == 0 || tab_keycode < keyboard->min_key_code || tab_keycode > keyboard->max_key_code)
+    goto out;
+
+  /* The keyboard geometry is stored by key "name" rather than keycode.
+   * (Key names are 4-character strings like like TAB or AE01.) We use the
+   * 'names' part of the keyboard description to map keycode to key name.
+   *
+   * XKB has a "key aliases" feature where a single keyboard key can have
+   * multiple names (with separate sets of aliases in the 'names' part and
+   * in the 'geometry' part), but I don't really understand it or how it is used,
+   * so I'm ignoring it here.
+   */
+
+  tab_name = keyboard->names->keys[tab_keycode].name; /* Not NULL terminated! */
+
+  /* First, iterate through the keyboard geometry to find the tab key; the keyboard
+   * geometry has a three-level heirarchy of section > row > key
+   */
+  for (i = 0; i < geometry->num_sections; i++)
+    {
+      XkbSectionPtr section = &geometry->sections[i];
+      for (j = 0; j < section->num_rows; j++)
+        {
+          int x = 0;
+          int y = 0;
+
+          XkbRowPtr row = &section->rows[j];
+          for (k = 0; k < row->num_keys; k++)
+            {
+              XkbKeyPtr key = &row->keys[k];
+              XkbShapePtr shape = XkbKeyShape (geometry, key);
+
+              if (row->vertical)
+                y += key->gap;
+              else
+                x += key->gap;
+
+              if (strncmp (key->name.name, tab_name, XkbKeyNameLength) == 0)
+                {
+                  tab_section = section;
+                  tab_bounds = shape->bounds;
+                  tab_bounds.x1 += row->left + x;
+                  tab_bounds.x2 += row->left + x;
+                  tab_bounds.y1 += row->top + y;
+                  tab_bounds.y2 += row->top + y;
+
+                  goto found_tab;
+                }
+
+              if (row->vertical)
+                y += (shape->bounds.y2 - shape->bounds.y1);
+              else
+                x += (shape->bounds.x2 - shape->bounds.x1);
+            }
+        }
+    }
+
+  /* No tab key found */
+  goto out;
+
+ found_tab:
+
+  /* Now find the key that:
+   *  - Is in the same section as the Tab key
+   *  - Has a horizontal center in the Tab key's horizonal bounds
+   *  - Is above the Tab key at a distance closer than any other key
+   *  - In case of ties, has its horizontal center as close as possible
+   *    to the Tab key's horizontal center
+   */
+  for (j = 0; j < tab_section->num_rows; j++)
+    {
+      int x = 0;
+      int y = 0;
+
+      XkbRowPtr row = &tab_section->rows[j];
+      for (k = 0; k < row->num_keys; k++)
+        {
+          XkbKeyPtr key = &row->keys[k];
+          XkbShapePtr shape = XkbKeyShape(geometry, key);
+          XkbBoundsRec bounds = shape->bounds;
+          int x_center;
+          int x_dist, y_dist;
+
+          if (row->vertical)
+            y += key->gap;
+          else
+            x += key->gap;
+
+          bounds.x1 += row->left + x;
+          bounds.x2 += row->left + x;
+          bounds.y1 += row->top + y;
+          bounds.y2 += row->top + y;
+
+          y_dist = tab_bounds.y1 - bounds.y2;
+          if (y_dist < 0)
+            continue;
+
+          x_center = (bounds.x1 + bounds.x2) / 2;
+          if (x_center < tab_bounds.x1 || x_center > tab_bounds.x2)
+            continue;
+
+          x_dist = ABS (x_center - (tab_bounds.x1 + tab_bounds.x2) / 2);
+
+          if (y_dist < best_y_dist ||
+              (y_dist == best_y_dist && x_dist < best_x_dist))
+            {
+              best_key = key;
+              best_x_dist = x_dist;
+              best_y_dist = y_dist;
+             }
+
+          if (row->vertical)
+            y += (shape->bounds.y2 - shape->bounds.y1);
+          else
+            x += (shape->bounds.x2 - shape->bounds.x1);
+        }
+    }
+
+  if (best_key == NULL)
+    goto out;
+
+  /* Now we need to resolve the name of the best key back to a keycode */
+  for (i = keyboard->min_key_code; i < keyboard->max_key_code; i++)
+    {
+      if (strncmp (best_key->name.name, keyboard->names->keys[i].name, XkbKeyNameLength) == 0)
+        {
+          best_keycode = i;
+          break;
+        }
+    }
+
+ out:
+  XkbFreeKeyboard (keyboard, 0, True);
+
+  return best_keycode;
+}
+#else /* !HAVE_XKB */
+static guint
+compute_above_tab_keycode (Display *xdisplay)
+{
+  return XKeysymToKeycode (xdisplay, XK_grave);
+}
+#endif /* HAVE_XKB */
+
+guint
+meta_display_get_above_tab_keycode (MetaDisplay *display)
+{
+  if (display->above_tab_keycode == 0) /* not yet computed */
+    display->above_tab_keycode = compute_above_tab_keycode (display->xdisplay);
+
+  if (display->above_tab_keycode == (guint)-1) /* failed to compute */
+    return 0;
+  else
+    return display->above_tab_keycode;
+}
diff --git a/src/core/display-private.h b/src/core/display-private.h
index 19287f36..cc304c33 100644
--- a/src/core/display-private.h
+++ b/src/core/display-private.h
@@ -207,6 +207,7 @@ struct _MetaDisplay
   KeySym *keymap;
   int keysyms_per_keycode;
   XModifierKeymap *modmap;
+  unsigned int above_tab_keycode;
   unsigned int ignored_modifier_mask;
   unsigned int num_lock_mask;
   unsigned int scroll_lock_mask;
@@ -497,4 +498,7 @@ void meta_display_queue_autoraise_callback  (MetaDisplay *display,
                                              MetaWindow  *window);
 void meta_display_remove_autoraise_callback (MetaDisplay *display);
 
+/* In above-tab-keycode.c */
+guint meta_display_get_above_tab_keycode (MetaDisplay *display);
+
 #endif
diff --git a/src/core/keybindings.c b/src/core/keybindings.c
index 1659c027..8ea6b415 100644
--- a/src/core/keybindings.c
+++ b/src/core/keybindings.c
@@ -138,6 +138,10 @@ reload_keymap (MetaDisplay *display)
   if (display->keymap)
     meta_XFree (display->keymap);
 
+  /* This is expensive to compute, so we'll lazily load if and when we first
+   * need it */
+  display->above_tab_keycode = 0;
+
   display->keymap = XGetKeyboardMapping (display->xdisplay,
                                          display->min_keycode,
                                          display->max_keycode -
@@ -249,6 +253,16 @@ reload_modmap (MetaDisplay *display)
               display->meta_mask);
 }
 
+static guint
+keysym_to_keycode (MetaDisplay *display,
+                   guint        keysym)
+{
+  if (keysym == META_KEY_ABOVE_TAB)
+    return meta_display_get_above_tab_keycode (display);
+  else
+    return XKeysymToKeycode (display->xdisplay, keysym);
+}
+
 static void
 reload_keycodes (MetaDisplay *display)
 {
@@ -264,8 +278,8 @@ reload_keycodes (MetaDisplay *display)
         {
           if (display->key_bindings[i].keysym != 0)
             {
-              display->key_bindings[i].keycode = XKeysymToKeycode (
-                      display->xdisplay, display->key_bindings[i].keysym);
+              display->key_bindings[i].keycode =
+                keysym_to_keycode (display, display->key_bindings[i].keysym);
             }
           
           ++i;
diff --git a/src/include/all-keybindings.h b/src/include/all-keybindings.h
index 378a7ac1..0759ec37 100644
--- a/src/include/all-keybindings.h
+++ b/src/include/all-keybindings.h
@@ -167,7 +167,7 @@ keybind (switch_panels_backward,   handle_switch,        META_TAB_LIST_DOCKS,
           "using a popup window"))
 
 keybind (cycle_group,               handle_cycle,         META_TAB_LIST_GROUP,
-        BINDING_REVERSES,        "<Alt>F6",
+        BINDING_REVERSES,        "<Alt>Above_Tab",
         _("Move between windows of an application immediately"))
 keybind (cycle_group_backward,     handle_cycle,         META_TAB_LIST_GROUP,
         REVERSES_AND_REVERSED,   NULL,
diff --git a/src/include/ui.h b/src/include/ui.h
index 6d98de8b..386581e7 100644
--- a/src/include/ui.h
+++ b/src/include/ui.h
@@ -185,6 +185,12 @@ void     meta_ui_set_current_theme (const char *name,
                                     gboolean    force_reload);
 gboolean meta_ui_have_a_theme      (void);
 
+/* Not a real key symbol but means "key above the tab key"; this is
+ * used as the default keybinding for cycle_group.
+ * 0x2xxxxxxx is a range not used by GDK or X. the remaining digits are
+ * randomly chosen */
+#define META_KEY_ABOVE_TAB 0x2f7259c9
+
 gboolean meta_ui_parse_accelerator (const char          *accel,
                                     unsigned int        *keysym,
                                     unsigned int        *keycode,
diff --git a/src/ui/ui.c b/src/ui/ui.c
index 960e7029..5ef4f525 100644
--- a/src/ui/ui.c
+++ b/src/ui/ui.c
@@ -896,14 +896,50 @@ meta_ui_accelerator_parse (const char      *accel,
                            guint           *keycode,
                            GdkModifierType *keymask)
 {
+  const char *above_tab;
+
   if (accel[0] == '0' && accel[1] == 'x')
     {
       *keysym = 0;
       *keycode = (guint) strtoul (accel, NULL, 16);
       *keymask = 0;
+
+      return;
     }
-  else
-    gtk_accelerator_parse (accel, keysym, keymask);
+
+  /* The key name 'Above_Tab' is special - it's not an actual keysym name,
+   * but rather refers to the key above the tab key. In order to use
+   * the GDK parsing for modifiers in combination with it, we substitute
+   * it with 'Tab' temporarily before calling gtk_accelerator_parse().
+   */
+#define is_word_character(c) (g_ascii_isalnum(c) || ((c) == '_'))
+#define ABOVE_TAB "Above_Tab"
+#define ABOVE_TAB_LEN 9
+
+  above_tab = strstr (accel, ABOVE_TAB);
+  if (above_tab &&
+      (above_tab == accel || !is_word_character (above_tab[-1])) &&
+      !is_word_character (above_tab[ABOVE_TAB_LEN]))
+    {
+      char *before = g_strndup (accel, above_tab - accel);
+      char *after = g_strdup (above_tab + ABOVE_TAB_LEN);
+      char *replaced = g_strconcat (before, "Tab", after, NULL);
+
+      gtk_accelerator_parse (replaced, NULL, keymask);
+
+      g_free (before);
+      g_free (after);
+      g_free (replaced);
+
+      *keysym = META_KEY_ABOVE_TAB;
+      return;
+    }
+
+#undef is_word_character
+#undef ABOVE_TAB
+#undef ABOVE_TAB_LEN
+
+  gtk_accelerator_parse (accel, keysym, keymask);
 }
 
 gboolean