Hello Dimitar ,
I’m able to reproduce the issue on mouse scroll :
AT the begnining, when the tree is not expanded , and only a horizontal scroll bar is required , the scroll is horizontal.
if you expand the tree, and now required a vertical scroll bar , The mouse scroll behaviour still horizontal.
In this example, there is no vertical scrollbar even after the expand.
If you set the “height” of the TreeGrid to 100 and you will see that when you expand the root record, vertical scrollbar is shown and mouse wheel controls the vertical scrollbar of the TreeGrid not the horizontal one.