Make the focused button to take the focus inside frame. (#371)

This resolves:
https://github.com/ArthurSonzogni/FTXUI/issues/370

examples/component/CMakeLists.txt

@@ -2,6 +2,7 @@ set(DIRECTORY_LIB component)
 
 example(button)
 example(button_animated)
+example(button_in_frame)
 example(button_style)
 example(canvas_animated)
 example(checkbox)

examples/component/button_in_frame.cpp

@@ -0,0 +1,48 @@
+#include <array>   // for array
+#include <memory>  // for shared_ptr, __shared_ptr_access
+#include <string>  // for operator+, to_string
+
+#include "ftxui/component/captured_mouse.hpp"  // for ftxui
+#include "ftxui/component/component.hpp"  // for Checkbox, Renderer, Vertical
+#include "ftxui/component/component_base.hpp"      // for ComponentBase
+#include "ftxui/component/screen_interactive.hpp"  // for ScreenInteractive
+#include "ftxui/dom/elements.hpp"  // for operator|, Element, size, border, frame, vscroll_indicator, HEIGHT, LESS_THAN
+
+using namespace ftxui;
+
+int main(int argc, const char* argv[]) {
+  int counter = 0;
+  auto on_click = [&] { counter++; };
+
+  auto button_style = ButtonOption::Animated(Color::Default, Color::GrayDark,
+                                             Color::Default, Color::White);
+
+  auto container = Container::Vertical({});
+  for (int i = 0; i < 30; ++i) {
+    auto button =
+        Button("Button " + std::to_string(i), on_click, &button_style);
+    container->Add(button);
+  }
+
+  auto renderer = Renderer(container, [&] {
+    return vbox({
+               hbox({
+                   text("Counter:"),
+                   text(std::to_string(counter)),
+               }),
+               separator(),
+               container->Render() | vscroll_indicator | frame |
+                   size(HEIGHT, LESS_THAN, 20),
+           }) |
+           border;
+  });
+
+  auto screen = ScreenInteractive::FitComponent();
+  screen.Loop(renderer);
+
+  return 0;
+}
+
+// Copyright 2022 Arthur Sonzogni. All rights reserved.
+// Use of this source code is governed by the MIT license that can be found in
+// the LICENSE file.