Fix WindowManager::GetCurrent() and the stderr it left behind on Linux

GetCurrent() asked the seat's keyboard for the window at its position, but that
call is about pointer position and GDK rejects keyboard devices, so the focused
window was never found and GDK logged an assertion failure instead
(libnativeapi/nativeapi-flutter#6). Walk the toplevels and prefer the active
one, which works on both X11 and Wayland.

The constructor also silenced GTK's headless warnings by reopening stderr onto
/dev/null and restoring it from /dev/tty. freopen() reuses the stream, so the
saved FILE* pointed at /dev/null too, and a process without a controlling
terminal — an SSH session, a systemd service, an app started from the desktop —
lost stderr for good. Swap the file descriptor instead and restore it from a
dup.

Author: lijy91

src/platform/linux/window_manager_linux.cpp

@@ -1,3 +1,5 @@
+#include <fcntl.h>
+#include <unistd.h>
 #include <cstring>
 #include <iostream>
 #include <mutex>
@@ -342,16 +344,25 @@ WindowManager::WindowManager() : pimpl_(std::make_unique<Impl>(this)) {
   // In headless environments, this may fail, which is acceptable
   if (!gdk_display_get_default()) {
     // Temporarily redirect stderr to suppress GTK warnings in headless
-    // environments
-    FILE* original_stderr = stderr;
-    freopen("/dev/null", "w", stderr);
+    // environments. This swaps the file descriptor rather than reopening the
+    // stderr stream: freopen() would leave the caller without a usable stderr
+    // whenever the process has no controlling terminal to restore from.
+    fflush(stderr);
+    int saved_stderr = dup(STDERR_FILENO);
+    int devnull = open("/dev/null", O_WRONLY | O_CLOEXEC);
+    if (devnull != -1) {
+      dup2(devnull, STDERR_FILENO);
+      close(devnull);
+    }
 
-    gboolean gtk_result = gtk_init_check(nullptr, nullptr);
+    gtk_init_check(nullptr, nullptr);
 
     // Restore stderr
     fflush(stderr);
-    freopen("/dev/tty", "w", stderr);
-    stderr = original_stderr;
+    if (saved_stderr != -1) {
+      dup2(saved_stderr, STDERR_FILENO);
+      close(saved_stderr);
+    }
 
     // gtk_init_check returns FALSE if initialization failed (e.g., no display)
     // This is acceptable for headless environments
@@ -514,34 +525,36 @@ std::shared_ptr<Window> WindowManager::GetCurrent() {
     return nullptr;
   }
 
-  // Try to get the focused window
-  GdkSeat* seat = gdk_display_get_default_seat(display);
-  if (seat) {
-    GdkDevice* keyboard = gdk_seat_get_keyboard(seat);
-    if (keyboard) {
-      GdkWindow* focused_window = gdk_device_get_window_at_position(keyboard, nullptr, nullptr);
-      if (focused_window) {
-        WindowId window_id = GetOrCreateWindowId(focused_window);
-        return Get(window_id);
-      }
-    }
-  }
-
-  // Fallback: get the first visible window
+  // The focused window, falling back to the first visible one. Asking the seat's
+  // keyboard for the window at its position is not an option: that call is about
+  // pointer position and GDK rejects keyboard devices outright, so on every
+  // backend it only logs an assertion failure.
+  GdkWindow* first_visible = nullptr;
   GList* toplevels = gtk_window_list_toplevels();
   for (GList* l = toplevels; l != nullptr; l = l->next) {
     GtkWindow* gtk_window = GTK_WINDOW(l->data);
-    if (gtk_widget_get_visible(GTK_WIDGET(gtk_window))) {
-      GdkWindow* gdk_window = gtk_widget_get_window(GTK_WIDGET(gtk_window));
-      if (gdk_window) {
-        WindowId window_id = GetOrCreateWindowId(gdk_window);
-        g_list_free(toplevels);
-        return Get(window_id);
-      }
+    if (!gtk_widget_get_visible(GTK_WIDGET(gtk_window))) {
+      continue;
+    }
+    GdkWindow* gdk_window = gtk_widget_get_window(GTK_WIDGET(gtk_window));
+    if (!gdk_window) {
+      continue;
+    }
+    if (gtk_window_is_active(gtk_window)) {
+      WindowId window_id = GetOrCreateWindowId(gdk_window);
+      g_list_free(toplevels);
+      return Get(window_id);
+    }
+    if (!first_visible) {
+      first_visible = gdk_window;
     }
   }
   g_list_free(toplevels);
 
+  if (first_visible) {
+    return Get(GetOrCreateWindowId(first_visible));
+  }
+
   return nullptr;
 }