Add additional coverage tests.

src/ftxui/dom/dbox_test.cpp

@@ -0,0 +1,40 @@
+#include <gtest/gtest-message.h>  // for Message
+#include <gtest/gtest-test-part.h>  // for SuiteApiResolver, TestFactoryImpl, TestPartResult
+#include <algorithm>                // for remove
+#include <string>                   // for allocator, basic_string, string
+#include <vector>                   // for vector
+
+#include "ftxui/dom/elements.hpp"  // for vtext, operator|, Element, flex_grow, flex_shrink, vbox
+#include "ftxui/dom/node.hpp"       // for Render
+#include "ftxui/screen/color.hpp"   // for ftxui
+#include "ftxui/screen/screen.hpp"  // for Screen
+#include "gtest/gtest_pred_impl.h"  // for Test, EXPECT_EQ, TEST
+
+namespace ftxui {
+
+TEST(DBoxTest, Basic) {
+  auto root = dbox({
+      hbox({
+          text("test") | border,
+          filler(),
+      }),
+      vbox({
+          text("test") | border,
+          filler(),
+      }),
+  });
+
+  Screen screen(8, 4);
+  Render(screen, root);
+  EXPECT_EQ(screen.ToString(),
+            "╭────┬─╮\r\n"
+            "│test│ │\r\n"
+            "╰────┴─╯\r\n"
+            "╰────╯  ");
+}
+
+}  // namespace ftxui
+
+// Copyright 2020 Arthur Sonzogni. All rights reserved.
+// Use of this source code is governed by the MIT license that can be found in
+// the LICENSE file.