ImGui::Begin("##whitewindow", nullptr, ImGuiWindowFlags_NoDecoration | ImGuiWindowFlags_NoBackground);
{
ImGui::SetWindowSize(ImVec2(300,300));
ImGui::GetWindowDrawList()->AddRectFilled(ImGui::GetWindowPos(), ImVec2(ImGui::GetWindowPos().x + 300, ImGui::GetWindowPos().x + 300), ImColor(255,255,255,255));
}ImGui::End();