Hello Sean,

The main aim of IgxCombo is to combine an input with IgxDropDown. As you know IgxDropDown is a component which displays a scrollable list of items. Looking at your question you need a drop down which should display a tree structure. If this is what you need you should not use IgxCombo, neither IgxDropDown as you do not need a drop down with scrollable list of trees. What you can use is IgxToggle directive. If you are using a button to open and close the drop down your markup should look something like this:

<button igxButton="raised" igxRipple [igxToggleAction]="toggle">Open tree grid</button>
<div igxToggle #toggle="toggle">
 <igx-tree-grid #grid1 [data]="data" childDataKey="childDataKey" primaryKey="ID" [width]="'900px'" [height]="'500px'">
  <!-- Add your columns here -->
 </igx-tree-grid>
</div>

Please let me know if this is what you need or if you have any additional questions on this matter.