Hello MS,
The main component and viewset is IBMAIN/IBMainViewSet. You can easily find it out by pressing on a button at the top along with F2. This viewset has 2 view areas you you can easiliy check what is their content and how IBHIER/TreeEnancedWindow is called. So the place where it is decided that the tree view is at the left and the other windows are at the right is IBMAIN/IBMainViewSet as I have explained already. So you could have a look at the HTML-coding or move the tree view window to the other view area (and maybe try to delete the left view area). But the easiest way without any enhancement is to click on "Increase Size" then the problem is solved for each user who does that.
Best regards,
Thomas