Hi santhosh,
Testing with the information that you provided I was unable to reproduce the issue as you do.
As far as I can comprehend from the shared info you have the smart-tree component in parent component and if the parent has a set height and the smart-tree component has overflowing items the scrollbar is visible.
I have a little code example:

<div class="tree-container">
   <smart-tree #tree id="tree" [scrollMode]="'scrollbar'" [selectionMode]="'oneOrMany'" [showLines]="true"></smart-tree>

And in the CSS:

  height: 200px;
smart-tree {
	width: 100%;
	height: 100%;

Let me know if that works for you but if this is not the case it will be better to share a bit more context of your situation!
Please, do not hesitate to contact us if you have any additional questions.
Best regards,
Yavor Dashev
Smart UI Team