Fix focus vs flexbox interaction. (#405)

- Fix focus in flexbox. This required resetting the focus state at the
  beginning of the ComputeRequirement(), because it can now run several
  times.

  This resolves:https://github.com/ArthurSonzogni/FTXUI/issues/399

- Add Box::Union.

- Add a preliminary implementation of forwarding selected_box from
  within the flexbox.

src/ftxui/dom/dbox.cpp

@@ -21,6 +21,7 @@ class DBox : public Node {
     requirement_.flex_grow_y = 0;
     requirement_.flex_shrink_x = 0;
     requirement_.flex_shrink_y = 0;
+    requirement_.selection = Requirement::NORMAL;
     for (auto& child : children_) {
       child->ComputeRequirement();
       requirement_.min_x =

src/ftxui/dom/flexbox.cpp

@@ -99,41 +99,63 @@ class Flexbox : public Node {
     }
     Layout(global, true);
 
+    // Reset:
+    requirement_.selection = Requirement::Selection::NORMAL;
+    requirement_.selected_box = Box();
+    requirement_.min_x = 0;
+    requirement_.min_y = 0;
+
     if (global.blocks.empty()) {
-      requirement_.min_x = 0;
-      requirement_.min_y = 0;
       return;
     }
 
+    // Compute the union of all the blocks:
     Box box;
     box.x_min = global.blocks[0].x;
     box.y_min = global.blocks[0].y;
     box.x_max = global.blocks[0].x + global.blocks[0].dim_x;
     box.y_max = global.blocks[0].y + global.blocks[0].dim_y;
-
     for (auto& b : global.blocks) {
       box.x_min = std::min(box.x_min, b.x);
       box.y_min = std::min(box.y_min, b.y);
       box.x_max = std::max(box.x_max, b.x + b.dim_x);
       box.y_max = std::max(box.y_max, b.y + b.dim_y);
     }
-
     requirement_.min_x = box.x_max - box.x_min;
     requirement_.min_y = box.y_max - box.y_min;
+
+    // Find the selection:
+    for (size_t i = 0; i < children_.size(); ++i) {
+      if (requirement_.selection >= children_[i]->requirement().selection) {
+        continue;
+      }
+      requirement_.selection = children_[i]->requirement().selection;
+      Box selected_box = children_[i]->requirement().selected_box;
+
+      // Shift |selected_box| according to its position inside this component:
+      auto& b = global.blocks[i];
+      selected_box.x_min += b.x;
+      selected_box.y_min += b.y;
+      selected_box.x_max += b.x;
+      selected_box.y_max += b.y;
+      requirement_.selected_box = Box::Intersection(selected_box, box);
+    }
   }
 
   void SetBox(Box box) override {
     Node::SetBox(box);
 
+    int asked_previous = asked_;
     asked_ = std::min(asked_, IsColumnOriented() ? box.y_max - box.y_min + 1
                                                  : box.x_max - box.x_min + 1);
+    need_iteration_ = (asked_ != asked_previous);
+
     flexbox_helper::Global global;
     global.config = config_;
     global.size_x = box.x_max - box.x_min + 1;
     global.size_y = box.y_max - box.y_min + 1;
     Layout(global);
 
-    need_iteration_ = false;
     for (size_t i = 0; i < children_.size(); ++i) {
       auto& child = children_[i];
       auto& b = global.blocks[i];

src/ftxui/dom/flexbox_test.cpp

@@ -432,6 +432,29 @@ TEST(FlexboxTest, GapY) {
             "       ");
 }
 
+TEST(FlexboxTest, Focus) {
+  auto document = vbox({
+    paragraph("0 -"),
+    paragraph("1 -"),
+    paragraph("2 -"),
+    paragraph("3 -"),
+    paragraph("4 -"),
+    paragraph("5 -"),
+    paragraph("6 -"),
+    paragraph("7 -") | focus,
+    paragraph("8 -"),
+    paragraph("9 -"),
+  }) | yframe | flex;
+
+  Screen screen(1, 3);
+  Render(screen, document);
+  EXPECT_EQ(screen.ToString(),
+            "7\r\n"
+            "-\r\n"
+            "8"
+            );
+}
+
 }  // namespace ftxui
 
 // Copyright 2021 Arthur Sonzogni. All rights reserved.

src/ftxui/dom/hbox.cpp

@@ -23,6 +23,7 @@ class HBox : public Node {
     requirement_.flex_grow_y = 0;
     requirement_.flex_shrink_x = 0;
     requirement_.flex_shrink_y = 0;
+    requirement_.selection = Requirement::NORMAL;
     for (auto& child : children_) {
       child->ComputeRequirement();
       if (requirement_.selection < child->requirement().selection) {

src/ftxui/dom/vbox.cpp

@@ -23,6 +23,7 @@ class VBox : public Node {
     requirement_.flex_grow_y = 0;
     requirement_.flex_shrink_x = 0;
     requirement_.flex_shrink_y = 0;
+    requirement_.selection = Requirement::NORMAL;
     for (auto& child : children_) {
       child->ComputeRequirement();
       if (requirement_.selection < child->requirement().selection) {