diff --git a/12to11-test.xml b/12to11-test.xml
index af8e62f..3fc6817 100644
--- a/12to11-test.xml
+++ b/12to11-test.xml
@@ -33,25 +33,46 @@
     <enum name="error">
       <entry name="role_present" value="1"
 	     summary="given wl_surface has/had another role"/>
+      <entry name="scale_lock_exists" value="2"
+	     summary="another client has already locked the scale"/>
+      <entry name="invalid_scale" value="3"
+	     summary="the specified scale is invalid"/>
     </enum>
 
     <request name="get_test_surface">
       <description summary="obtain test surface role">
 	Get a test_surface object for a particular surface.  If a role
 	was already attached to this surface, or a role of a different
-	type was previously attached, a role_present error is issued.
+	type was previously attached, post a role_present error.
 
-	The window is created immediately after get_test_surface is
-	called.  It is mapped once a commit request with a non-nil
-	buffer is made.
+	Create the window immediately after get_test_surface is
+	called; the window is mapped once a commit request with a
+	non-nil buffer is made.
 
 	Once the window associated with the test_surface object is
-	mapped, a mapped event is sent.
+	mapped, send a mapped event.
       </description>
       <arg name="id" type="new_id" interface="test_surface"/>
       <arg name="surface" type="object" interface="wl_surface"/>
     </request>
 
+    <request name="get_scale_lock">
+      <description summary="obtain scale lock">
+	Create a new test_scale_lock object.  As long as this object
+	exists, set the global output scale to the scale specified in
+	this request (or in a subsequent test_scale_lock.set_scale
+	request.)
+
+	If a test_scale_lock object already exists, generate a
+	scale_lock_exists error.
+
+	If the scale is invalid (by being less than 0), post an
+	invalid_scale error.
+      </description>
+      <arg name="id" type="new_id" interface="test_scale_lock"/>
+      <arg name="global_scale" type="uint"/>
+    </request>
+
     <event name="display_string">
       <description summary="X server name">
 	The display_string event sends the name of the X display to
@@ -74,8 +95,8 @@
 
     <request name="destroy" type="destructor">
       <description summary="destroy role">
-	This request destroys the test_surface role.  Subsequently,
-	get_test_surface can be called again with its surface.
+	Destroy the test_surface role.  Subsequently, get_test_surface
+	can be called again with its surface.
       </description>
     </request>
 
@@ -94,4 +115,29 @@
       <arg name="display_string" type="string"/>
     </event>
   </interface>
+
+  <interface name="test_scale_lock" version="1">
+    <description summary="test scale lock">
+      A scale lock resource represents the "lock" a client holds on
+      the global output scale.  While it still exists, the scale for
+      every output will be whatever was specified when the lock was
+      created or by the last set_scale request.
+    </description>
+
+    <request name="destroy" type="destructor">
+      <description summary="destroy scale lock">
+	Destroy the specified scale lock, restore the actual scale of
+	each output, and let clients call get_scale_lock again.
+      </description>
+    </request>
+
+    <request name="set_scale">
+      <description summary="set the global output scale">
+	Set the scale of each output to the specified scale, and
+	resize all surfaces accordingly.  If the scale is invalid,
+	post an invalid_scale error.
+      </description>
+      <arg name="scale" type="uint"/>
+    </request>
+  </interface>
 </protocol>
diff --git a/compositor.h b/compositor.h
index f9ee30d..e43f4d1 100644
--- a/compositor.h
+++ b/compositor.h
@@ -1206,6 +1206,7 @@ extern void XLRemoveScaleChangeCallback (void *);
 extern void XLClearOutputs (Surface *);
 extern void XLOutputSetChangeFunction (void (*) (Time));
 extern void XLGetMaxOutputBounds (int *, int *, int *, int *);
+extern void XLOutputHandleScaleChange (int);
 
 /* Defined in atoms.c.  */
 
@@ -1799,6 +1800,8 @@ extern void XLInitPointerGestures (void);
 
 /* Defined in test.c.  */
 
+extern int locked_output_scale;
+
 extern void XLInitTest (void);
 extern Bool XLHandleOneXEventForTest (XEvent *);
 
diff --git a/output.c b/output.c
index d8402fb..351f1ea 100644
--- a/output.c
+++ b/output.c
@@ -98,6 +98,9 @@ static ScaleChangeCallback scale_callbacks;
 /* The scale factor currently applied on a global basis.  */
 int global_scale_factor;
 
+/* The current real scale factor.  */
+static int real_scale_factor;
+
 /* Function run upon any kind of XRandR notify event.  */
 static void (*change_hook) (Time);
 
@@ -1035,6 +1038,19 @@ HandleScaleChange (int scale)
   RunScaleChangeCallbacks ();
 }
 
+static void
+HandleScaleSettingChange (int scale)
+{
+  real_scale_factor = scale;
+
+  /* If the scale factor is locked, just return.  */
+  if (locked_output_scale)
+    return;
+
+  /* Otherwise, update the scale.  */
+  HandleScaleChange (scale);
+}
+
 void *
 XLAddScaleChangeCallback (void *data, void (*func) (void *, int))
 {
@@ -1142,6 +1158,18 @@ XLGetMaxOutputBounds (int *x_min, int *y_min, int *x_max,
   *y_max = y2;
 }
 
+void
+XLOutputHandleScaleChange (int new_scale)
+{
+  /* If the value is not -1, just use the new scale.
+     Otherwise, use the current non-locked scale.  */
+
+  if (new_scale != -1)
+    HandleScaleChange (new_scale);
+  else
+    HandleScaleChange (real_scale_factor);
+}
+
 void
 XLInitRROutputs (void)
 {
@@ -1179,7 +1207,10 @@ XLInitRROutputs (void)
      factor was specified for debugging.  */
   if (!ApplyEnvironment ("GLOBAL_SCALE", &global_scale_factor))
     XLListenToIntegerSetting ("Gdk/WindowScalingFactor",
-			      HandleScaleChange);
+			      HandleScaleSettingChange);
+
+  /* Set the real scale factor.  */
+  real_scale_factor = global_scale_factor;
 
   /* Initialize the scale change callback list sentinel node.  */
   scale_callbacks.next = &scale_callbacks;
@@ -1204,4 +1235,3 @@ XLInitRROutputs (void)
   all_outputs = BuildOutputTree ();
   MakeGlobalsForOutputTree (all_outputs);
 }
-
diff --git a/test.c b/test.c
index 8e36110..ed78ad3 100644
--- a/test.c
+++ b/test.c
@@ -60,6 +60,11 @@ struct _TestSurface
   int bounds_width, bounds_height;
 };
 
+/* The locked output scale.  N.B. that a test_scale_lock is not an
+   actual resource, and just represents the state of this
+   variable.  */
+int locked_output_scale;
+
 /* The test surface manager global.  */
 static struct wl_global *test_manager_global;
 
@@ -348,6 +353,47 @@ HandleResourceDestroy (struct wl_resource *resource)
   DestroyBacking (test);
 }
 
+
+
+static void
+DestroyScaleLock (struct wl_client *client, struct wl_resource *resource)
+{
+  wl_resource_destroy (resource);
+}
+
+static void
+SetScale (struct wl_client *client, struct wl_resource *resource,
+	  uint32_t scale)
+{
+  /* If the scale is invalid, reject it.  */
+  if (!scale)
+    {
+      wl_resource_post_error (resource, TEST_MANAGER_ERROR_INVALID_SCALE,
+			      "scale of 0 specified");
+      return;
+    }
+
+  /* Set the scale.  As there can only be one lock at any given
+     time, there is no need to check the resource data.  */
+  locked_output_scale = scale;
+  XLOutputHandleScaleChange (scale);
+}
+
+static const struct test_scale_lock_interface scale_lock_impl =
+  {
+    .destroy = DestroyScaleLock,
+    .set_scale = SetScale,
+  };
+
+static void
+HandleScaleLockResourceDestroy (struct wl_resource *resource)
+{
+  /* There is no resource data associated with scale locks.  Just
+     unlock the scale.  */
+  locked_output_scale = 0;
+  XLOutputHandleScaleChange (-1);
+}
+
 static void
 GetTestSurface (struct wl_client *client, struct wl_resource *resource,
 		uint32_t id, struct wl_resource *surface_resource)
@@ -451,9 +497,55 @@ GetTestSurface (struct wl_client *client, struct wl_resource *resource,
     abort ();
 }
 
+static void
+GetScaleLock (struct wl_client *client, struct wl_resource *resource,
+	      uint32_t id, uint32_t scale)
+{
+  struct wl_resource *lock_resource;
+
+  if (!scale)
+    {
+      wl_resource_post_error (resource, TEST_MANAGER_ERROR_INVALID_SCALE,
+			      "scale of 0 specified");
+      return;
+    }
+
+  if (locked_output_scale)
+    {
+      /* The scale is already locked, so don't create another
+	 lock.  */
+      wl_resource_post_error (resource, TEST_MANAGER_ERROR_SCALE_LOCK_EXISTS,
+			      "a scale lock already exists (another test is"
+			      " already running?)");
+      return;
+    }
+
+  lock_resource = wl_resource_create (client, &test_scale_lock_interface,
+				      wl_resource_get_version (resource),
+				      id);
+
+  if (!lock_resource)
+    {
+      wl_resource_post_no_memory (resource);
+      return;
+    }
+
+  /* Now, set the locked scale.  */
+  locked_output_scale = scale;
+
+  /* And update the global scale factor if need be.  */
+  if (scale != global_scale_factor)
+    XLOutputHandleScaleChange (scale);
+
+  /* And resource implementation.  */
+  wl_resource_set_implementation (lock_resource, &scale_lock_impl,
+				  NULL, HandleScaleLockResourceDestroy);
+}
+
 static const struct test_manager_interface test_manager_impl =
   {
     .get_test_surface = GetTestSurface,
+    .get_scale_lock = GetScaleLock,
   };
 
 
diff --git a/tests/Imakefile b/tests/Imakefile
index 48871fd..f521a9d 100644
--- a/tests/Imakefile
+++ b/tests/Imakefile
@@ -2,6 +2,7 @@
 
      12TO11ROOT = ..
         DEPLIBS = $(DEPXLIB)
+  SYS_LIBRARIES = MathLibrary
 LOCAL_LIBRARIES = $(WAYLAND_CLIENT) $(XLIB) $(PNG)
      COMMONOBJS = test_harness.o
      COMMONSRCS = test_harness.c
@@ -35,7 +36,9 @@ ScannerTarget(viewporter)
           OBJS5 = $(COMMONSRCS) viewporter_test.o
           SRCS6 = $(COMMONSRCS) subsurface_test.c
           OBJS6 = $(COMMONSRCS) subsurface_test.o
-       PROGRAMS = imgview simple_test damage_test transform_test viewporter_test subsurface_test
+          SRCS7 = $(COMMONSRCS) scale_test.c
+          OBJS7 = $(COMMONSRCS) scale_test.o
+       PROGRAMS = imgview simple_test damage_test transform_test viewporter_test subsurface_test scale_test
 
 /* Make all objects depend on HEADER.  */
 $(OBJS1): $(HEADER)
@@ -49,7 +52,9 @@ NormalProgramTarget(damage_test,$(OBJS3),NullParameter,$(LOCAL_LIBRARIES),NullPa
 NormalProgramTarget(transform_test,$(OBJS4),NullParameter,$(LOCAL_LIBRARIES),NullParameter)
 NormalProgramTarget(viewporter_test,$(OBJS5),NullParameter,$(LOCAL_LIBRARIES),NullParameter)
 NormalProgramTarget(subsurface_test,$(OBJS6),NullParameter,$(LOCAL_LIBRARIES),NullParameter)
+NormalProgramTarget(scale_test,$(OBJS7),NullParameter,$(LOCAL_LIBRARIES),NullParameter)
 DependTarget3($(SRCS1),$(SRCS2),$(SRCS3))
 DependTarget3($(SRCS4),$(SRCS5),$(SRCS6))
+DependTarget3($(SRCS7),NullParameter,NullParameter)
 
 all:: $(PROGRAMS)
diff --git a/tests/run_tests.sh b/tests/run_tests.sh
index e93bb87..80a79a0 100755
--- a/tests/run_tests.sh
+++ b/tests/run_tests.sh
@@ -21,14 +21,11 @@
 pushd "$(dirname $0)"
 declare -a standard_tests=(
     simple_test damage_test transform_test viewporter_test
-    subsurface_test
+    subsurface_test scale_test
 )
 
 make -C . "${standard_tests[@]}"
 
-# Run tandard tests, meaning those that should be run with a
-# GLOBAL_SCALE of 1 and an OUTPUT_SCALE of 1.
-
 export GLOBAL_SCALE=1
 export OUTPUT_SCALE=1
 exec 3< <(stdbuf -oL ../12to11 -printsocket)
diff --git a/tests/svnignore.txt b/tests/svnignore.txt
index a006657..e0d4c69 100644
--- a/tests/svnignore.txt
+++ b/tests/svnignore.txt
@@ -6,6 +6,7 @@ damage_test
 transform_test
 viewporter_test
 subsurface_test
+scale_test
 imgview
 reject.dump
 Makefile
diff --git a/tests/test_harness.c b/tests/test_harness.c
index 5c084a7..f89ebd3 100644
--- a/tests/test_harness.c
+++ b/tests/test_harness.c
@@ -45,6 +45,9 @@ along with 12to11.  If not, see <https://www.gnu.org/licenses/>.  */
 /* Whether or not to write image data instead of verifying it.  */
 static bool write_image_data_instead;
 
+/* The test display.  */
+static struct test_display *display;
+
 static void
 handle_test_manager_display_string (void *data, struct test_manager *manager,
 				    const char *display_string)
@@ -154,10 +157,29 @@ die (const char *reason)
   exit (1);
 }
 
+/* This should be called when a test failed due to a non-IO error.  It
+   destroys the scale lock and synchronizes with the compositor so
+   that by the time the next test is run, the current test's scale
+   lock will have been released.  */
+
+static void __attribute__ ((noreturn))
+exit_with_code (int code)
+{
+  if (display)
+    {
+      test_scale_lock_destroy (display->scale_lock);
+      wl_display_roundtrip (display->display);
+    }
+
+  exit (code);
+}
+
 struct test_display *
 open_test_display (struct test_interface *interfaces, int num_interfaces)
 {
-  struct test_display *display;
+  if (display)
+    /* The display was already opened.  */
+    return display;
 
   display = malloc (sizeof *display);
 
@@ -191,8 +213,17 @@ open_test_display (struct test_interface *interfaces, int num_interfaces)
   if (!test_manager_check (display))
     goto error_3;
 
+  /* And try to set up the scale lock.  */
+  display->scale_lock
+    = test_manager_get_scale_lock (display->test_manager, 1);
+
+  if (!display->scale_lock)
+    goto error_4;
+
   return display;
 
+ error_4:
+  XFree (display->pixmap_formats);
  error_3:
   if (display->x_display)
     XCloseDisplay (display->x_display);
@@ -200,6 +231,7 @@ open_test_display (struct test_interface *interfaces, int num_interfaces)
   wl_display_disconnect (display->display);
  error_1:
   free (display);
+  display = NULL;
   return NULL;
 }
 
@@ -325,7 +357,7 @@ report_test_failure (const char *format, ...)
   fputs ("\n", stderr);
   va_end (ap);
 
-  exit (1);
+  exit_with_code (1);
 }
 
 void __attribute__ ((format (gnu_printf, 1, 2)))
@@ -766,6 +798,12 @@ verify_image_data (struct test_display *display, Window window,
   test_log ("verified image data");
 }
 
+void
+test_set_scale (struct test_display *display, int scale)
+{
+  test_scale_lock_set_scale (display->scale_lock, scale);
+}
+
 void
 test_init (void)
 {
@@ -777,5 +815,5 @@ void __attribute__ ((noreturn))
 test_complete (void)
 {
   test_log ("test ran successfully");
-  exit (0);
+  exit_with_code (0);
 }
diff --git a/tests/test_harness.h b/tests/test_harness.h
index d95bfc8..aa54a03 100644
--- a/tests/test_harness.h
+++ b/tests/test_harness.h
@@ -51,6 +51,9 @@ struct test_display
   struct wl_shm *shm;
   struct test_manager *test_manager;
 
+  /* The test scale lock.  */
+  struct test_scale_lock *scale_lock;
+
   /* Test interfaces.  */
   struct test_interface *interfaces;
 
@@ -119,6 +122,7 @@ extern unsigned char *load_image_data (const char *,
 extern void verify_image_data (struct test_display *, Window, const char *);
 extern void test_init (void);
 extern void test_complete (void) __attribute__ ((noreturn));
+extern void test_set_scale (struct test_display *, int);
 
 #define ARRAYELTS(arr) (sizeof (arr) / sizeof (arr)[0])