When the tree is collapsed in Sample-> 18_subtasks_displaying, subtasks are displayed in one line. However, sub milestones are shown as tasks when the tree is collapsed. Can’t I get milestones to be displayed as they are?

Hello Seungmin,
In that sample, the subtasks are displayed with the help of the additional task layer:
To make it work as you expect, you just need to turn on the open_split_tasks parameter:
And here is the sample: