bool BeginCombo(const char* label, const char* preview_value, ImGuiComboFlags flags = 0);
void EndCombo(); // only call EndCombo() if BeginCombo() returns true!

const char* items[] = { "AAAA", "BBBB", "CCCC", "DDDD", "EEEE", "FFFF", "GGGG", "HHHH", "IIII", "JJJJ", "KKKK", "LLLLLLL", "MMMM", "OOOOOOO", "PPPP", "QQQQQQQQQQ", "RRR", "SSSS" };
static const char* current_item = NULL;
if (ImGui::BeginCombo("##combo", current_item)) // The second parameter is the label previewed before opening the combo.
{
    for (int n = 0; n < IM_ARRAYSIZE(items); n++)
    {
        bool is_selected = (current_item == items[n]); // You can store your selection however you want, outside or inside your objects
        if (ImGui::Selectable(items[n], is_selected)
            current_item = items[n];
        if (is_selected)
            ImGui::SetItemDefaultFocus();   // You may set the initial focus when opening the combo (scrolling + for keyboard navigation support)
    }
    ImGui::EndCombo();
}

const char* items[] = { "AAAA", "BBBB", "CCCC", "DDDD", "EEEE", "FFFF", "GGGG", "HHHH", "IIII", "JJJJ", "KKKK", "LLLLLLL", "MMMM", "OOOOOOO", "PPPP", "QQQQQQQQQQ", "RRR", "SSSS" };
static const char* current_item = NULL;
ImGuiComboFlags flags = ImGuiComboFlags_NoArrowButton;
ImGuiStyle& style = ImGui::GetStyle();
float w = ImGui::CalcItemWidth();
float spacing = style.ItemInnerSpacing.x;
float button_sz = ImGui::GetFrameHeight();
ImGui::PushItemWidth(w - spacing * 2.0f - button_sz * 2.0f);
if (ImGui::BeginCombo("##custom combo", current_item, ImGuiComboFlags_NoArrowButton))
{
    for (int n = 0; n < IM_ARRAYSIZE(items); n++)
    {
        bool is_selected = (current_item == items[n]);
        if (ImGui::Selectable(items[n], is_selected))
            current_item = items[n];
        if (is_selected)
            ImGui::SetItemDefaultFocus();
    }
    ImGui::EndCombo();
}
ImGui::PopItemWidth();
ImGui::SameLine(0, spacing);
if (ImGui::ArrowButton("##r", ImGuiDir_Left))
{
}
ImGui::SameLine(0, spacing);
if (ImGui::ArrowButton("##r", ImGuiDir_Right))
{
}
ImGui::SameLine(0, style.ItemInnerSpacing.x);
ImGui::Text("Custom Combo");