diff --git a/tests/select_helper.c b/tests/select_helper.c
new file mode 100644
index 0000000..a2cdf70
--- /dev/null
+++ b/tests/select_helper.c
@@ -0,0 +1,201 @@
+/* Tests for the Wayland compositor running on the X server.
+
+Copyright (C) 2022 to various contributors.
+
+This file is part of 12to11.
+
+12to11 is free software: you can redistribute it and/or modify it
+under the terms of the GNU General Public License as published by the
+Free Software Foundation, either version 3 of the License, or (at your
+option) any later version.
+
+12to11 is distributed in the hope that it will be useful, but WITHOUT
+ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+for more details.
+
+You should have received a copy of the GNU General Public License
+along with 12to11.  If not, see <https://www.gnu.org/licenses/>.  */
+
+#include <stdbool.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+
+#include <X11/Xlib.h>
+#include <X11/Xatom.h>
+
+/* select_helper.c -- Read the data of the clipboard selection and
+   print them to stdout.
+
+   There must be three arguments: the name of the display, the
+   timestamp at which the selection was acquired, and the target.  */
+
+
+/* The display connected to.  */
+static Display *display;
+
+/* The selection transfer window.  */
+static Window selection_transfer_window;
+
+/* Various atoms.  */
+static Atom CLIPBOARD, target_atom, INCR;
+
+static void
+wait_for_selection_notify (XEvent *event)
+{
+  while (true)
+    {
+      XNextEvent (display, event);
+
+      if (event->type == SelectionNotify
+	  && (event->xselection.requestor
+	      == selection_transfer_window)
+	  && (event->xselection.selection
+	      == CLIPBOARD)
+	  && (event->xselection.property
+	      == target_atom)
+	  && (event->xselection.target
+	      == target_atom))
+	return;
+    }
+}
+
+static void
+wait_for_new_value (XEvent *event, Atom property)
+{
+  while (true)
+    {
+      XNextEvent (display, event);
+
+      if (event->type == PropertyNotify
+	  && event->xproperty.atom == property
+	  && event->xproperty.state == PropertyNewValue)
+	return;
+    }
+}
+
+static size_t
+get_size_for_format (int format)
+{
+  switch (format)
+    {
+    case 32:
+      return sizeof (long);
+
+    case 16:
+      return sizeof (short int);
+
+    case 8:
+      return sizeof (char);
+    }
+
+  /* Should not actually happen.  */
+  return 0;
+}
+
+int
+main (int argc, char **argv)
+{
+  XSetWindowAttributes attrs;
+  unsigned long flags, timestamp;
+  char *atom_names[2];
+  Atom atoms[3], actual_type, property;
+  XEvent event;
+  int actual_format;
+  unsigned long nitems, bytes_after;
+  unsigned char *data;
+
+  if (argc < 4)
+    /* Not enough arguments were specified.  */
+    return 1;
+
+  display = XOpenDisplay (argv[1]);
+
+  if (!display)
+    return 1;
+
+  /* Make the window used to transfer selection data.  */
+  attrs.override_redirect = True;
+  attrs.event_mask = PropertyChangeMask;
+  flags = CWEventMask | CWOverrideRedirect;
+
+  selection_transfer_window
+    = XCreateWindow (display, DefaultRootWindow (display),
+		     -1, -1, 1, 1, 0, CopyFromParent, InputOnly,
+		     CopyFromParent, flags, &attrs);
+
+  /* Get the time.  */
+  timestamp = strtoul (argv[2], NULL, 10);
+
+  atom_names[0] = argv[3];
+  atom_names[1] = (char *) "CLIPBOARD";
+  atom_names[2] = (char *) "CLIPBOARD";
+  XInternAtoms (display, atom_names, 3, False, atoms);
+  target_atom = atoms[0];
+  CLIPBOARD = atoms[1];
+  INCR = atoms[2];
+
+  /* Now ask for CLIPBOARD.  */
+  XConvertSelection (display, CLIPBOARD, target_atom,
+		     target_atom, selection_transfer_window,
+		     timestamp);
+
+  /* And wait for the SelectionNotify event.  */
+  wait_for_selection_notify (&event);
+
+  /* Selection conversion failed.  */
+  if (event.xselection.property == None)
+    return 1;
+
+  property = event.xselection.property;
+
+  XGetWindowProperty (display, selection_transfer_window,
+		      property, 0, 0xffffffff, True, AnyPropertyType,
+		      &actual_type, &actual_format, &nitems, &bytes_after,
+		      &data);
+
+  if (!data || bytes_after)
+    return 1;
+
+  if (actual_type == INCR)
+    {
+      while (true)
+	{
+	  XFree (data);
+
+	  wait_for_new_value (&event, property);
+	  XGetWindowProperty (display, selection_transfer_window, property, 0,
+			      0xffffffff, True, AnyPropertyType, &actual_type,
+			      &actual_format, &nitems, &bytes_after, &data);
+
+	  if (!data)
+	    return 0;
+
+	  if (nitems)
+	    {
+	      /* Write the selection data to stdout.  */
+	      if (fwrite (data, get_size_for_format (actual_format),
+			  nitems, stdout) != nitems)
+		return 1;
+
+	      continue;
+	    }
+
+	  /* Selection transfer is complete.  */
+	  fflush (stdout);
+	  return 0;
+	}
+    }
+  else
+    {
+      /* Write the selection data to stdout.  */
+      if (fwrite (data, get_size_for_format (actual_format),
+		  nitems, stdout) != nitems)
+	return 1;
+
+      /* Return success.  */
+      fflush (stdout);
+      return 0;
+    }
+}
diff --git a/tests/select_test.c b/tests/select_test.c
new file mode 100644
index 0000000..626c9b8
--- /dev/null
+++ b/tests/select_test.c
@@ -0,0 +1,311 @@
+/* Tests for the Wayland compositor running on the X server.
+
+Copyright (C) 2022 to various contributors.
+
+This file is part of 12to11.
+
+12to11 is free software: you can redistribute it and/or modify it
+under the terms of the GNU General Public License as published by the
+Free Software Foundation, either version 3 of the License, or (at your
+option) any later version.
+
+12to11 is distributed in the hope that it will be useful, but WITHOUT
+ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+for more details.
+
+You should have received a copy of the GNU General Public License
+along with 12to11.  If not, see <https://www.gnu.org/licenses/>.  */
+
+#include <pthread.h>
+#include <sys/wait.h>
+
+#include "test_harness.h"
+
+#include <X11/Xatom.h>
+
+enum test_kind
+  {
+    SELECT_STRING_KIND,
+  };
+
+static const char *test_names[] =
+  {
+    "select_string",
+  };
+
+#define LAST_TEST	        SELECT_STRING_KIND
+
+/* The display.  */
+static struct test_display *display;
+
+/* The data device manager.  */
+static struct wl_data_device_manager *data_device_manager;
+
+/* Test interfaces.  */
+static struct test_interface test_interfaces[] =
+  {
+    { "wl_data_device_manager", &data_device_manager,
+      &wl_data_device_manager_interface, 3, },
+  };
+
+/* The data device.  */
+static struct wl_data_device *data_device;
+
+/* Whether or not the data source's send listener was called.  */
+static bool send_called;
+
+/* Sample text used.  */
+
+#define SAMPLE_TEXT							\
+  ("Lorem ipsum dolor sit amet, consectetur adipiscing elit"		\
+   ", sed do eiusmod tempor incididunt ut labore et dolore"		\
+   " magna aliqua. Ut enim ad minim veniam, quis nostrud"		\
+   " exercitation ullamco laboris nisi ut aliquip ex ea commodo"	\
+   " consequat.  Duis aute irure dolor in reprehenderit in"		\
+   " voluptate velit esse cillum dolore eu fugiat nulla pariatur."	\
+   "  Excepteur sint occaecat cupidatat non proident, sunt in"		\
+   " culpa qui officia deserunt mollit anim id est laborum.")
+
+
+
+static Bool
+test_get_time_1 (Display *display, XEvent *event, XPointer arg)
+{
+  Atom *atom;
+
+  atom = (Atom *) arg;
+
+  if (event->type == PropertyNotify
+      && event->xproperty.atom == *atom)
+    return True;
+
+  return False;
+}
+
+/* Get a timestamp suitable for use in events dispatched to the test
+   seat.  */
+
+static Time
+test_get_time (void)
+{
+  Atom property_atom;
+  XEvent event;
+  Window window;
+  unsigned char unused;
+  XSetWindowAttributes attrs;
+
+  attrs.event_mask = PropertyChangeMask;
+  window = XCreateWindow (display->x_display,
+			  DefaultRootWindow (display->x_display),
+			  0, 0, 1, 1, 0, 0, InputOnly, CopyFromParent,
+			  CWEventMask, &attrs);
+  unused = '\0';
+  property_atom = XInternAtom (display->x_display,
+			       "_INTERNAL_SERVER_TIME_PROP",
+			       False);
+  XChangeProperty (display->x_display, window, property_atom,
+		   XA_CARDINAL, 8, PropModeReplace, &unused, 1);
+  XIfEvent (display->x_display, &event, test_get_time_1,
+	    (XPointer) &property_atom);
+  XDestroyWindow (display->x_display, window);
+  return event.xproperty.time;
+}
+
+
+
+static void
+handle_data_source_target (void *data, struct wl_data_source *data_source,
+			   const char *mime_type)
+{
+  /* Nothing to do here.  */
+}
+
+static void *
+handle_data_source_send_1 (void *data)
+{
+  int fd;
+
+  fd = (intptr_t) data;
+
+  write (fd, SAMPLE_TEXT, sizeof SAMPLE_TEXT - 1);
+  close (fd);
+
+  return NULL;
+}
+
+static void
+handle_data_source_send (void *data, struct wl_data_source *data_source,
+			 const char *mime_type, int fd)
+{
+  pthread_t thread;
+
+  send_called = true;
+
+  if (pthread_create (&thread, NULL, handle_data_source_send_1,
+		      (void *) (intptr_t) fd))
+    die ("pthread_create");
+}
+
+static void
+handle_data_source_cancelled (void *data, struct wl_data_source *data_source)
+{
+  report_test_failure ("data source cancelled");
+}
+
+static const struct wl_data_source_listener data_source_listener =
+  {
+    handle_data_source_target,
+    handle_data_source_send,
+    handle_data_source_cancelled,
+  };
+
+
+
+static void
+own_sample_text (void)
+{
+  struct wl_data_source *source;
+  uint32_t display_serial;
+
+  source = wl_data_device_manager_create_data_source (data_device_manager);
+  display_serial = test_get_serial (display);
+
+  if (!source)
+    report_test_failure ("failed to create data source");
+
+  wl_data_source_offer (source, "text/plain");
+  wl_data_source_offer (source, "text/plain;charset=utf-8");
+  wl_data_source_add_listener (source, &data_source_listener, NULL);
+  wl_data_device_set_selection (data_device, source, display_serial);
+}
+
+static void
+verify_sample_text (Time time)
+{
+  int pipefds[2], wstatus;
+  pid_t pid;
+  char *display_string;
+  char time_buffer[45], buffer[sizeof SAMPLE_TEXT];
+  ssize_t bytes_read;
+  const char *sample_text_buffer;
+
+  /* Run select_helper with the specified timestamp.  Wait until
+     handle_data_source_send is called, and then begin reading from
+     the pipe.  */
+
+  if (pipe (pipefds) < 0)
+    die ("pipe");
+
+  display_string = DisplayString (display->x_display);
+  time = sprintf (time_buffer, "%lu", time);
+  pid = fork ();
+
+  if (pid == -1)
+    die ("fork");
+  else if (!pid)
+    {
+      close (pipefds[0]);
+
+      if (!dup2 (pipefds[1], 1))
+	exit (1);
+
+      execlp ("./select_helper", "./select_helper",
+	      display_string, time_buffer, "STRING",
+	      NULL);
+      exit (1);
+    }
+
+  send_called = false;
+
+  while (!send_called)
+    {
+      if (wl_display_dispatch (display->display) == -1)
+        die ("wl_display_dispatch");
+    }
+
+  /* Now, start reading from the pipe and comparing the contents.  */
+  sample_text_buffer = SAMPLE_TEXT;
+  bytes_read = read (pipefds[0], buffer, sizeof SAMPLE_TEXT - 1);
+
+  if (bytes_read != sizeof SAMPLE_TEXT - 1)
+    report_test_failure ("wanted %zu bytes, but got %zd",
+			 sizeof SAMPLE_TEXT - 1, bytes_read);
+
+  waitpid (pid, &wstatus, 0);
+
+  if (WEXITSTATUS (wstatus))
+    report_test_failure ("child exited with failure: %d",
+			 WEXITSTATUS (wstatus));
+
+  /* Now compare the text.  */
+  if (memcmp (buffer, sample_text_buffer, bytes_read))
+    report_test_failure ("read text differs from sample text!");
+
+  close (pipefds[0]);
+  close (pipefds[1]);
+}
+
+
+
+static void
+test_single_step (enum test_kind kind)
+{
+  Time time;
+
+  test_log ("running test step: %s", test_names[kind]);
+
+  switch (kind)
+    {
+    case SELECT_STRING_KIND:
+      /* Set the last user time of the seat to the current X server
+	 time.  */
+      time = test_get_time ();
+      test_seat_controller_set_last_user_time (display->seat->controller,
+					       0, time);
+      own_sample_text ();
+
+      /* Do a roundtrip.  If selection ownership changes, then the
+	 protocol translator will wait for selection ownership to be
+	 confirmed.  */
+      wl_display_roundtrip (display->display);
+
+      /* Now, verify the selection contents.  */
+      verify_sample_text (time);
+      break;
+    }
+
+  if (kind == LAST_TEST)
+    test_complete ();
+}
+
+
+
+static void
+run_test (void)
+{
+  test_single_step (SELECT_STRING_KIND);
+
+  while (true)
+    {
+      if (wl_display_dispatch (display->display) == -1)
+        die ("wl_display_dispatch");
+    }
+}
+
+int
+main (void)
+{
+  test_init ();
+  display = open_test_display (test_interfaces,
+			       ARRAYELTS (test_interfaces));
+
+  if (!display)
+    report_test_failure ("failed to open display");
+
+  test_init_seat (display);
+  data_device
+    = wl_data_device_manager_get_data_device (data_device_manager,
+					      display->seat->seat);
+  run_test ();
+}