How to add a custom dropdown to the quill toolbar?

I’ve been trying to add a custom dropdown in the toolbar without any luck so far. There is a similar question here quilljs/quill#1817 but the proposed solution did not worked out for me (and it seems a bit strange). I have already customized my toolbar with some custom buttons like this:

<quill-editor #editor [formControl]="body" (onEditorCreated)="onEditorCreated($event)">
  <div quill-editor-toolbar>

    <!-- Basic buttons -->
    <span class="ql-formats">
      <button class="ql-bold" [title]="'Bold'"></button>
      <button class="ql-italic" [title]="'Italic'"></button>
      <button class="ql-underline" [title]="'Underline'"></button>
      <button class="ql-strike" [title]="'Strike'"></button>
    </span>

    <!-- Size -->
    <span class="ql-formats">
      <select class="ql-size" [title]="'Size'">
        <option value="small"></option>
        <option selected></option>
        <option value="large"></option>
        <option value="huge"></option>
      </select>
    </span>

    <!-- Custom actions -->
    <span class="ql-formats">
      <button [title]="'Signature'" (click)="onAddSignature()">
        <mat-icon fontSet="fontawesome" fontIcon="fa-pencil"></mat-icon>
      </button>
      <button [title]="'Unsubscribe'" (click)="onAddUnsubscribeLink()">
        <mat-icon fontSet="fontawesome" fontIcon="fa-optin-monster"></mat-icon>
      </button>
    </span>

  </div>
</quill-editor>

But when I try to add a custom dropdown using the markup, the dropdown is shown without any options. To provide a bit more context I am trying to achieve a line spacing solution similar to this: https://github.com/quilljs/quill/issues/197#issuecomment-306092948