x11: Avoid excessive keymap reconstruction

KeymapNotify events happen on focus events, as well as when the keymap group changes. Query the current group and don't rebuild the keymap if it hasn't changed.

Note that some IME changes, such as activating intelligent Japanese or Chinese input methods on Gnome, will only trigger IBus activation, and won't send a keymap or group update as they use the existing layout.

src/video/x11/SDL_x11events.c

@@ -1131,7 +1131,17 @@ static void X11_DispatchEvent(SDL_VideoDevice *_this, XEvent *xevent)
             printf("window %p: KeymapNotify!\n", data);
 #endif
             if (SDL_GetKeyboardFocus() != NULL) {
-                X11_UpdateKeymap(_this, SDL_TRUE);
+#ifdef SDL_VIDEO_DRIVER_X11_HAS_XKBLOOKUPKEYSYM
+                if (videodata->xkb) {
+                    XkbStateRec state;
+                    if (X11_XkbGetState(videodata->display, XkbUseCoreKbd, &state) == Success) {
+                        if (state.group != videodata->xkb_group) {
+                            /* Only rebuild the keymap if the layout has changed. */
+                            X11_UpdateKeymap(_this, SDL_TRUE);
+                        }
+                    }
+                }
+#endif
                 X11_ReconcileKeyboardState(_this);
             }
         } else if (xevent->type == MappingNotify) {

src/video/x11/SDL_x11keyboard.c

@@ -347,7 +347,6 @@ void X11_UpdateKeymap(SDL_VideoDevice *_this, SDL_bool send_event)
     int i;
     SDL_Scancode scancode;
     SDL_Keymap *keymap;
-    unsigned char group = 0;
 
     keymap = SDL_CreateKeymap();
 
@@ -357,7 +356,7 @@ void X11_UpdateKeymap(SDL_VideoDevice *_this, SDL_bool send_event)
         X11_XkbGetUpdatedMap(data->display, XkbAllClientInfoMask, data->xkb);
 
         if (X11_XkbGetState(data->display, XkbUseCoreKbd, &state) == Success) {
-            group = state.group;
+            data->xkb_group = state.group;
         }
     }
 #endif
@@ -372,7 +371,7 @@ void X11_UpdateKeymap(SDL_VideoDevice *_this, SDL_bool send_event)
                 continue;
             }
 
-            KeySym keysym = X11_KeyCodeToSym(_this, i, group, keymod_masks[m].xkb_mask);
+            KeySym keysym = X11_KeyCodeToSym(_this, i, data->xkb_group, keymod_masks[m].xkb_mask);
 
             /* Note: The default SDL scancode table sets this to right alt instead of AltGr/Mode, so handle it separately. */
             if (keysym != XK_ISO_Level3_Shift) {

src/video/x11/SDL_x11video.h

@@ -125,6 +125,7 @@ struct SDL_VideoData
     XkbDescPtr xkb;
 #endif
     int xkb_event;
+    unsigned int xkb_group;
 
     KeyCode filter_code;
     Time filter_time;