diff --git a/src/backend/wayland/wl_backend.c b/src/backend/wayland/wl_backend.c
index 588f48a..50e360c 100644
--- a/src/backend/wayland/wl_backend.c
+++ b/src/backend/wayland/wl_backend.c
@@ -468,6 +468,9 @@ frame_timer_cb(void *data)
 #define VIEW_FROM_LISTENER(ptr, field) \
     ((struct wl_toplevel_view *)((char *)(ptr) - offsetof(struct wl_toplevel_view, field)))
 
+/* X error handler — defined near the vtable at the bottom of this file. */
+static int wl_catch_xwayland_error(Display *edpy, XErrorEvent *error);
+
 /* Phase 43: forward declaration — defined in the focus section below. */
 static void wl_focus_set(WNativeWindow win, int revert_to, unsigned long timestamp);
 /* Phase 50: forward declaration — defined in client section below. */
@@ -1394,7 +1397,10 @@ handle_backend_new_input(struct wl_listener *listener, void *data)
             wlr_keyboard_set_keymap(kb, wl_state.xkb_keymap);
         wl_state.real_keyboard_dev = dev;
         wl_state.keyboard_dev      = dev;
-        wlr_seat_set_keyboard(wl_state.seat, kb);
+        /* Seat may not exist yet if new_input fires during wlr_backend_start()
+         * before wlr_seat_create() is called later in wl_display_open(). */
+        if (wl_state.seat)
+            wlr_seat_set_keyboard(wl_state.seat, kb);
         wl_state.real_keyboard_key_listener.notify = handle_keyboard_key;
         wl_signal_add(&kb->events.key,
                       &wl_state.real_keyboard_key_listener);
@@ -1484,13 +1490,24 @@ handle_backend_new_output(struct wl_listener *listener, void *data)
     if (wl_state.allocator && wl_state.renderer)
         wlr_output_init_render(output, wl_state.allocator, wl_state.renderer);
 
-    /* Enable the output: select preferred mode, then commit via state API. */
+    /* Enable the output: select preferred mode, then commit via state API.
+     * Nested Wayland outputs: sway's initial xdg-shell configure sends 0×0
+     * ("compositor defers to client"), so output->width/height stay 0 after
+     * the configure busy-loop.  Without an explicit mode the swapchain tries
+     * to allocate a 0×0 GBM buffer which crashes.  Fall back to 1920×1080 so
+     * the commit succeeds; sway will send a real configure once the window is
+     * visible and we handle it in the frame/configure signal path. */
     {
         struct wlr_output_state state;
         wlr_output_state_init(&state);
         wlr_output_state_set_enabled(&state, true);
         struct wlr_output_mode *mode = wlr_output_preferred_mode(output);
-        if (mode) wlr_output_state_set_mode(&state, mode);
+        if (mode) {
+            wlr_output_state_set_mode(&state, mode);
+        } else {
+            /* Default size for nested mode (or headless with no preferred mode). */
+            wlr_output_state_set_custom_mode(&state, 1920, 1080, 0);
+        }
         if (!wlr_output_commit_state(output, &state))
             wwarning("wl_backend: initial output commit failed");
         wlr_output_state_finish(&state);
@@ -1557,6 +1574,14 @@ wl_display_open(const char *name)
 
     (void)name; /* Wayland compositor picks its own socket name */
 
+    /* Install X error handler immediately: Mesa/EGL may open a connection to
+     * DISPLAY (e.g. the host compositor's XWayland) during wlr_backend_start,
+     * which happens before StartUp() calls register_error_handlers().
+     * Installing here prevents Xlib's default handler from aborting the
+     * process on the benign BadMatch that XWayland generates on SetInputFocus
+     * during WM startup. */
+    XSetErrorHandler(wl_catch_xwayland_error);
+
     wlr_log_init(WLR_ERROR, NULL);
 
     wl_state.display = wl_display_create();