This is a usability bug. Technically it's working as designed, but it could be easier and less frustrating.

Steps to reproduce the issue

These steps talk about custom fields, but it also applies to the core subform field.

1. Create 5 custom fields, set to subform only. (Alternatively: only two custom fields, and one of them is an editor field.)
2. Create a subform custom field and add all custom fields to the subform. Make sure repeatable is set to yes.
3. Navigate to an article where you can edit the subform custom field.
4. Add 2-3 rows.
5. Try to reorder the rows.

Expected result

You can clearly see where the row started and where you are putting it in the context of other rows.

Actual result

So. Much. Green. You cannot always clearly see where things are going or where you started, or drag it into positions that are not visible on the screen.

System information (as much as possible)

N/A, this has been an issue for a while.

Additional comments

A possible solution is collapsing rows into "ghost" representative rows while dragging so you can see more at once, and using a more minimal drop indicator instead of a full-height item.

This happens even when you don't have too many fields, especially if one of the fields is an editor field or an accessible media field, so it's not necessarily a matter of too many sub fields in a row.