backend/xcursor: Support a "blank" cursor type

We don't have enough Xlib code in mutter ...

Joking aside, it can be useful to make the cursor invisible
without hiding it, for example for replacing the actual cursor
with an actor in gnome-shell; the real cursor should still
update the focus surface in that case, and not sneak into
screenshots or -casts.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1244

src/backends/meta-cursor-sprite-xcursor.c

@@ -84,6 +84,7 @@ translate_meta_cursor (MetaCursor cursor)
       return "crosshair";
     case META_CURSOR_IBEAM:
       return "xterm";
+    case META_CURSOR_BLANK:
     case META_CURSOR_NONE:
     case META_CURSOR_LAST:
       break;
@@ -93,6 +94,48 @@ translate_meta_cursor (MetaCursor cursor)
   return NULL;
 }
 
+static Cursor
+create_blank_cursor (Display *xdisplay)
+{
+  Pixmap pixmap;
+  XColor color;
+  Cursor cursor;
+  XGCValues gc_values;
+  GC gc;
+
+  pixmap = XCreatePixmap (xdisplay, DefaultRootWindow (xdisplay), 1, 1, 1);
+
+  gc_values.foreground = BlackPixel (xdisplay, DefaultScreen (xdisplay));
+  gc = XCreateGC (xdisplay, pixmap, GCForeground, &gc_values);
+
+  XFillRectangle (xdisplay, pixmap, gc, 0, 0, 1, 1);
+
+  color.pixel = 0;
+  color.red = color.blue = color.green = 0;
+
+  cursor = XCreatePixmapCursor (xdisplay, pixmap, pixmap, &color, &color, 1, 1);
+
+  XFreeGC (xdisplay, gc);
+  XFreePixmap (xdisplay, pixmap);
+
+  return cursor;
+}
+
+static XcursorImages *
+create_blank_cursor_images (void)
+{
+  XcursorImages *images;
+
+  images = XcursorImagesCreate (1);
+  images->images[0] = XcursorImageCreate (1, 1);
+
+  images->images[0]->xhot = 0;
+  images->images[0]->yhot = 0;
+  memset (images->images[0]->pixels, 0, sizeof(int32_t));
+
+  return images;
+}
+
 MetaCursor
 meta_cursor_sprite_xcursor_get_cursor (MetaCursorSpriteXcursor *sprite_xcursor)
 {
@@ -103,12 +146,18 @@ Cursor
 meta_create_x_cursor (Display    *xdisplay,
                       MetaCursor  cursor)
 {
+  if (cursor == META_CURSOR_BLANK)
+    return create_blank_cursor (xdisplay);
+
   return XcursorLibraryLoadCursor (xdisplay, translate_meta_cursor (cursor));
 }
 
 static XcursorImages *
 load_cursor_on_client (MetaCursor cursor, int scale)
 {
+  if (cursor == META_CURSOR_BLANK)
+    return create_blank_cursor_images ();
+
   return XcursorLibraryLoadImages (translate_meta_cursor (cursor),
                                    meta_prefs_get_cursor_theme (),
                                    meta_prefs_get_cursor_size () * scale);

src/meta/common.h

@@ -204,6 +204,7 @@ typedef enum
  * @META_CURSOR_POINTING_HAND: pointing hand
  * @META_CURSOR_CROSSHAIR: crosshair (action forbidden)
  * @META_CURSOR_IBEAM: I-beam (text input)
+ * @META_CURSOR_BLANK: Invisible cursor
  */
 typedef enum
 {
@@ -226,6 +227,7 @@ typedef enum
   META_CURSOR_POINTING_HAND,
   META_CURSOR_CROSSHAIR,
   META_CURSOR_IBEAM,
+  META_CURSOR_BLANK,
   META_CURSOR_LAST
 } MetaCursor;