const WindowId kNullWindowId = 0;
-typedef int ScreenId;
+// Type used to identify screens on the desktop. Values are platform-specific:
+// - On Windows: integer display device index.
+// - On OSX: CGDirectDisplayID cast to intptr_t.
+// - On Linux (with X11): TBD.
+typedef intptr_t ScreenId;
// The screen id corresponds to all screen combined together.
const ScreenId kFullDesktopScreenId = -1;