Use IWYU.

include/ftxui/component/checkbox.hpp

@@ -1,11 +1,13 @@
 #ifndef FTXUI_COMPONENT_CHECKBOX_HPP
 #define FTXUI_COMPONENT_CHECKBOX_HPP
 
-#include <functional>
+#include <string>
 
 #include "ftxui/component/component.hpp"
+#include "ftxui/screen/box.hpp"
 
 namespace ftxui {
+struct Event;
 
 /// @brief A Checkbox. It can be checked or unchecked.Display an element on a
 /// ftxui::Screen.
@@ -42,7 +44,6 @@ class CheckBox : public Component {
 
   int cursor_position = 0;
   Box box_;
-
 };
 
 }  // namespace ftxui