static ImGuiDockNodeFlags dockspace_flags = ImGuiDockNodeFlags_PassthruCentralNode;

// We are using the ImGuiWindowFlags_NoDocking flag to make the parent window not dockable into,
// because it would be confusing to have two docking targets within each others.
ImGuiWindowFlags window_flags = ImGuiWindowFlags_MenuBar | ImGuiWindowFlags_NoDocking;

ImGuiViewport* viewport = ImGui::GetMainViewport();
ImGui::SetNextWindowPos(viewport->Pos);
ImGui::SetNextWindowSize(viewport->Size);
ImGui::SetNextWindowViewport(viewport->ID);
ImGui::PushStyleVar(ImGuiStyleVar_WindowRounding, 0.0f);
ImGui::PushStyleVar(ImGuiStyleVar_WindowBorderSize, 0.0f);
window_flags |= ImGuiWindowFlags_NoTitleBar | ImGuiWindowFlags_NoCollapse | ImGuiWindowFlags_NoResize | ImGuiWindowFlags_NoMove;
window_flags |= ImGuiWindowFlags_NoBringToFrontOnFocus | ImGuiWindowFlags_NoNavFocus;


// When using ImGuiDockNodeFlags_PassthruCentralNode, DockSpace() will render our background and handle the pass-thru hole, so we ask Begin() to not render a background.
if (dockspace_flags & ImGuiDockNodeFlags_PassthruCentralNode)
window_flags |= ImGuiWindowFlags_NoBackground;

// Important: note that we proceed even if Begin() returns false (aka window is collapsed).
// This is because we want to keep our DockSpace() active. If a DockSpace() is inactive, 
// all active windows docked into it will lose their parent and become undocked.
// We cannot preserve the docking relationship between an active window and an inactive docking, otherwise 
// any change of dockspace/settings would lead to windows being stuck in limbo and never being visible.
ImGui::PushStyleVar(ImGuiStyleVar_WindowPadding, ImVec2(0.0f, 0.0f));
ImGui::Begin("DockSpace", nullptr, window_flags);
ImGui::PopStyleVar();
ImGui::PopStyleVar(2);

// DockSpace
ImGuiIO& io = ImGui::GetIO();
if (io.ConfigFlags & ImGuiConfigFlags_DockingEnable)
{
	ImGuiID dockspace_id = ImGui::GetID("MyDockSpace");
	ImGui::DockSpace(dockspace_id, ImVec2(0.0f, 0.0f), dockspace_flags);

	static auto first_time = true;
	if (first_time)
	{
		first_time = false;

		ImGui::DockBuilderRemoveNode(dockspace_id); // clear any previous layout
		ImGui::DockBuilderAddNode(dockspace_id, dockspace_flags | ImGuiDockNodeFlags_DockSpace);
		ImGui::DockBuilderSetNodeSize(dockspace_id, viewport->Size);

		// split the dockspace into 2 nodes -- DockBuilderSplitNode takes in the following args in the following order
		//   window ID to split, direction, fraction (between 0 and 1), the final two setting let's us choose which id we want (which ever one we DON'T set as NULL, will be returned by the function)
		//                                                              out_id_at_dir is the id of the node in the direction we specified earlier, out_id_at_opposite_dir is in the opposite direction
		auto dock_id_left = ImGui::DockBuilderSplitNode(dockspace_id, ImGuiDir_Left, 0.2f, nullptr, &dockspace_id);
		auto dock_id_down = ImGui::DockBuilderSplitNode(dockspace_id, ImGuiDir_Down, 0.25f, nullptr, &dockspace_id);

		// we now dock our windows into the docking node we made above
		ImGui::DockBuilderDockWindow("Down", dock_id_down);
		ImGui::DockBuilderDockWindow("Left", dock_id_left);
		ImGui::DockBuilderFinish(dockspace_id);
	}
}

ImGui::End();

ImGui::Begin("Left");
ImGui::Text("Hello, left!");
ImGui::End();

ImGui::Begin("Down");
ImGui::Text("Hello, down!");
ImGui::End();