Minimal reproducer for gtk3/mutter bug https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=3961

app.c

#include <gtk/gtk.h>
#include <gdk/gdk.h>
#include <stdlib.h>

static void
activate (GtkApplication* app,
          gpointer        user_data)
{
  (void)user_data;
  GdkScreen *screen;
  GtkWidget *window;
  GtkWidget *button;
  GtkWidget *button_box;
  GdkRectangle viewable_area = { 0 };
  int x;
  int width;
  int height;

  screen = gdk_screen_get_default ();
  if (!screen) {
    abort();
  }
  /* Hard-coded for second screen! */
  gdk_screen_get_monitor_geometry (screen, 1, &viewable_area);
  x = viewable_area.x;
  width = viewable_area.width;
  height = viewable_area.height;
  printf("x=%d width=%d height=%d\n", x, width, height);

  window = gtk_application_window_new (app);
  gtk_window_set_title (GTK_WINDOW (window), "Window");
  gtk_window_move (GTK_WINDOW (window), x, 0);
  gtk_window_set_default_size (GTK_WINDOW (window), width, height);

  button_box = gtk_button_box_new (GTK_ORIENTATION_HORIZONTAL);
  gtk_widget_set_valign (button_box, GTK_ALIGN_START);
  gtk_container_add (GTK_CONTAINER (window), button_box);
  button = gtk_button_new_with_label ("Hello World");
  gtk_container_add (GTK_CONTAINER (button_box), button);

  gtk_widget_show_all (window);
}

int
main (int    argc,
      char **argv)
{
  GtkApplication *app;
  int status;

  app = gtk_application_new ("org.gtk.example", G_APPLICATION_FLAGS_NONE);
  g_signal_connect (app, "activate", G_CALLBACK (activate), NULL);
  status = g_application_run (G_APPLICATION (app), argc, argv);
  g_object_unref (app);

  return status;
}
/* vim: set sw=2 ts=2 et: */

Makefile

CFLAGS = -Wall -Wextra -g $(shell pkg-config --cflags gtk+-3.0)
LDFLAGS = $(shell pkg-config --libs gtk+-3.0)

app: app.c
	$(CC) $(CFLAGS) $< -o $@ $(LDFLAGS)

diff --git a/app.c b/app.c
index f6ede76..60fa81a 100644
--- a/app.c
+++ b/app.c
@@ -11,6 +11,7 @@ activate (GtkApplication* app,
   GtkWidget *window;
   GtkWidget *button;
   GtkWidget *button_box;
+  GdkRectangle viewable_area;
   int x;
   int height;

@@ -18,14 +19,15 @@ activate (GtkApplication* app,
   if (!screen) {
     abort();
   }
-  x = gdk_screen_get_width (screen);
-  height = gdk_screen_get_height (screen);
+  gdk_screen_get_monitor_geometry (screen, 1, &viewable_area);
+  x = viewable_area.x;
+  height = viewable_area.height;
   printf("x=%d height=%d\n", x, height);

   window = gtk_application_window_new (app);
   gtk_window_set_title (GTK_WINDOW (window), "Window");
   gtk_window_move (GTK_WINDOW (window), x, 0);
-  gtk_window_set_default_size (GTK_WINDOW (window), 200, height);
+  gtk_window_set_default_size (GTK_WINDOW (window), x, height);

   button_box = gtk_button_box_new (GTK_ORIENTATION_HORIZONTAL);
   gtk_widget_set_valign (button_box, GTK_ALIGN_START);