Class ParamDragPreview

src/components/params_editor.rs:397–400

                                        .on_click({
                                            let this = this.clone();
                                            move |_, window, cx| {
                                                this.update(cx, |editor, cx| {
                                                    editor.add_param(window, cx);
                                                });
                                            }
                                        }),
                                )