Continuing the discussion from WebDialog centered on screen or SU main window:
Only on the PC editions. (On Mac these class window are just “always on top”, but do not hold the focus until dismissed as they do under Windows.)
See the [UI::WebDialog#show_modal()] (Class: UI::WebDialog — SketchUp Ruby API Documentation) instance method
The new SketchUp 2017+ UI::HTMLDialog#show_modal() instance method may work on both platforms. (There is no exception for Mac OSX is the API documentation at least. So test to be sure.)