Prevent drop-down-item to scroll over search text

I would like to find a way how I can keep my search text displayed in a dropdown box and not scrolled over by the dropdown items. In my case i have a dropdown box that has on top a searchbox that allows user to filter the list to narrow down the items. The filter etc works fine, the only issue i am having is that once the user starts scrolling it will scroll over the search box. How could i achieve that ?

This is what the dropdown looks before scroll

Then once user enters a search and filter it looks like this

Here is the HTM Code

<div class="inputs">
    <button igxButton (click)="toggleDropDown($event)"
            type="button"
            [class]="buttonClass"
            [igxDropDownItemNavigation]="dropdown1">
        {{title}}
        <igx-icon class="material-icons igx-icon" aria-hidden="true">arrow_drop_down</igx-icon>
    </button>

    <igx-drop-down #dropdown1 (selectionChanging)="onSelect($event)" #toggleDropdownMenu>
        <div class="drop-down__scroll-container filter-input">
            <igx-input-group  [ngStyle]="{'display': showSearch ? 'block' : 'none' }">
                <input igxInput #optionsearch
                       class="search-input"
                       type="text"
                       id="search"
                >
                <igx-suffix>
                    <igx-icon (click)="refreshDropDown()">refresh</igx-icon>
                    <igx-icon (click)="clearSearch()">clear</igx-icon>
                </igx-suffix>
                <label igxLabel for="search">Search</label>
            </igx-input-group>


            <igx-drop-down-item *ngFor="let option of options |  optionFilter : optionsearch?.value : 'name'"
                                [value]="option">
                {{ option.name }}
            </igx-drop-down-item>
        </div>
    </igx-drop-down>
</div>

And the CSS Style for the Componenet

button {
    border: 1px solid;
    margin-bottom: 0;
}

.drop-down__scroll-container {
    height: 300px;
    padding-top: 16px;
    overflow: scroll;
}

::ng-deep {
    .filter-input {
        .igx-input-group {
            position: sticky;
            top: -16px;
            background-color: white;
            padding: 11px 22px;
            -webkit-box-shadow: 0px 7px 20px 0px rgba(0, 0, 0, 0.26);
            -moz-box-shadow: 0px 7px 20px 0px rgba(0, 0, 0, 0.26);
            box-shadow: 0px 7px 20px 0px rgba(0, 0, 0, 0.26);
        }

        .igx-input-group__bundle:hover {
            box-shadow: none
        }

        .igx-input-group__bundle {
            padding-top: 0 !important;
            box-shadow: none;
        }
    }

}

You are doing a custom styling that is not complying with the default z-index styles. In order to overcome this problem you can set a higher z-index to the input-group component:

This is StackBlitz example based on the app structure that you showed