x11: Synthesize fullscreen size events on Openbox

Openbox doesn't send size events when entering fullscreen, so they must be synthesized. This is not desirable on any other window manager, as it can break fullscreen positioning on multi-monitor configurations.

src/video/x11/SDL_x11video.c

@@ -78,6 +78,23 @@ static bool X11_IsXWayland(Display *d)
     return X11_XQueryExtension(d, "XWAYLAND", &opcode, &event, &error) == True;
 }
 
+static bool X11_CheckCurrentDesktop(const char *name)
+{
+    SDL_Environment *env = SDL_GetEnvironment();
+
+    const char *desktopVar = SDL_GetEnvironmentVariable(env, "DESKTOP_SESSION");
+    if (desktopVar && SDL_strcasecmp(desktopVar, name) == 0) {
+        return true;
+    }
+
+    desktopVar = SDL_GetEnvironmentVariable(env, "XDG_CURRENT_DESKTOP");
+    if (desktopVar && SDL_strcasestr(desktopVar, name)) {
+        return true;
+    }
+
+    return false;
+}
+
 static SDL_VideoDevice *X11_CreateDevice(void)
 {
     SDL_VideoDevice *device;
@@ -256,8 +273,14 @@ static SDL_VideoDevice *X11_CreateDevice(void)
         device->system_theme = SDL_SystemTheme_Get();
 #endif
 
-    device->device_caps = VIDEO_DEVICE_CAPS_HAS_POPUP_WINDOW_SUPPORT |
-                          VIDEO_DEVICE_CAPS_SENDS_FULLSCREEN_DIMENSIONS;
+    device->device_caps = VIDEO_DEVICE_CAPS_HAS_POPUP_WINDOW_SUPPORT;
+
+    /* Openbox doesn't send the new window dimensions when entering fullscreen, so the events must be synthesized.
+     * This is otherwise not wanted, as it can break fullscreen window positioning on multi-monitor configurations.
+     */
+    if (!X11_CheckCurrentDesktop("openbox")) {
+        device->device_caps |= VIDEO_DEVICE_CAPS_SENDS_DISPLAY_CHANGES;
+    }
 
     data->is_xwayland = X11_IsXWayland(x11_display);
     if (data->is_xwayland) {