Allow window id to be passed to ImageGrab.grab() on macOS

docs/reference/ImageGrab.rst

@@ -44,9 +44,11 @@ or the clipboard to a PIL image memory.
         .. versionadded:: 7.1.0
 
     :param window:
-        HWND, to capture a single window. Windows only.
+        Capture a single window. On Windows, this is a HWND. On macOS, this is a
+        CGWindowID.
 
-        .. versionadded:: 11.2.1
+        .. versionadded:: 11.2.1 Windows support
+        .. versionadded:: 12.0.0 macOS support
     :return: An image
 
 .. py:function:: grabclipboard()

src/PIL/ImageGrab.py

@@ -43,17 +43,45 @@ def grab(
             fh, filepath = tempfile.mkstemp(".png")
             os.close(fh)
             args = ["screencapture"]
-            if bbox:
+            if window:
+                args += ["-l", str(window)]
+            elif bbox:
                 left, top, right, bottom = bbox
                 args += ["-R", f"{left},{top},{right-left},{bottom-top}"]
             subprocess.call(args + ["-x", filepath])
             im = Image.open(filepath)
             im.load()
             os.unlink(filepath)
             if bbox:
-                im_resized = im.resize((right - left, bottom - top))
-                im.close()
-                return im_resized
+                if window:
+                    # Determine if the window was in retina mode or not
+                    # by capturing it without the shadow,
+                    # and checking how different the width is
+                    fh, filepath = tempfile.mkstemp(".png")
+                    os.close(fh)
+                    subprocess.call(
+                        ["screencapture", "-l", str(window), "-o", "-x", filepath]
+                    )
+                    with Image.open(filepath) as im_no_shadow:
+                        retina = im.width - im_no_shadow.width > 100
+                    os.unlink(filepath)
+
+                    # Since screencapture's -R does not work with -l,
+                    # crop the image manually
+                    if retina:
+                        left, top, right, bottom = bbox
+                        im_cropped = im.resize(
+                            (right - left, bottom - top),
+                            box=tuple(coord * 2 for coord in bbox),
+                        )
+                    else:
+                        im_cropped = im.crop(bbox)
+                    im.close()
+                    return im_cropped
+                else:
+                    im_resized = im.resize((right - left, bottom - top))
+                    im.close()
+                    return im_resized
             return im
         elif sys.platform == "win32":
             if window is not None: