Initial content
@await FlushAsync()
Secondary content
@await FlushAsync()
@{
    await Html.RenderPartialAsync("_PartialWithFlush");
}
@using (Html.BeginForm(method: FormMethod.Post, antiforgery: false, htmlAttributes: null))
{
    @Html.TextBox("Name1")
    @await FlushAsync()
    @Html.TextBox("Name2")
}
