Add hflow.

examples/dom/size.cpp

@@ -0,0 +1,30 @@
+#include "ftxui/screen/screen.hpp"
+#include "ftxui/screen/string.hpp"
+#include "ftxui/dom/elements.hpp"
+#include <iostream>
+
+int main(int argc, const char *argv[])
+{
+  using namespace ftxui;
+  auto make_box = [](const std::wstring title) {
+    return
+      window(
+        text(title) | hcenter | bold,
+        text(L"content") | hcenter | dim
+      );
+  };
+
+  Elements content;
+  for(int x = 3; x<30; ++x) {
+    content.push_back(
+      make_box(to_wstring(x))
+        | size(WIDTH, EQUAL, x)
+    );
+  }
+  auto document = hbox(std::move(content));
+  auto screen = Screen::FitDocument(document);
+  Render(screen, document.get());
+  std::cout << screen.ToString() << std::endl;
+
+  return 0;
+}