C++ ImGui Drag and drop item inside collapsing header

imgui_drag_and_drop.cpp

if (ImGui::CollapsingHeader(group.name.c_str()))
{    
    float window_width = ImGui::GetWindowPos().x + ImGui::GetWindowContentRegionMax().x;
    size_t buttonCounts = group.apps.size();

    for (int index = 0; index < buttonCounts; ++index)
    {
        ImGui::PushID(index);

        ImGui::Button(group.apps[index].name.c_str(), button_sz);

        // HANDLE DRAG AND DROP BUTTON
        // Our buttons are both drag sources and drag targets
        if (ImGui::BeginDragDropSource())
        {
            // Set payload to carry the index of our button
            ImGui::SetDragDropPayload("DND_BUTTON", &index, sizeof(int));

            // Display preview
            ImGui::Text("Moving %s", group.apps[index].name.c_str());

            ImGui::EndDragDropSource();
        }

        if (ImGui::BeginDragDropTarget())
        {
            if (const ImGuiPayload* payload = ImGui::AcceptDragDropPayload("DND_BUTTON"))
            {
                int payload_n = *(const int*)payload->Data;

                std::swap(group.apps[index], group.apps[payload_n]);
            }

            ImGui::EndDragDropTarget();
        }

        ImGui::PopID();
    }
}