“(i) You replied on …”
The system already adds “(i) You replied on Fri 2019-10-18 7:45 PM” as a header to messages replied to; and that is a wonderful addition.
I would find it helpful to have that “You replied on…” system message hyperlinked to the indicated reply message in whatever folder it's in.
This would prove helpful when one accidentally misfiles a reply message, or when moving original and reply messages from the “Inbox” and “Sent” folders to the same, or different, folders (Allowing: Move original message from Inbox, Click "You replied on…", Drag message from “Sent” folder to same destination folder).
Ideally, the left-most “Folders” scrollbar would remain in the same location (showing the last destination folder), while the folder highlighting would change (to the “Sent” folder), while the folder contents (the middle column of wide display format) of the messages in "Sent" would scroll to display the highlighted message, allowing easy dragging to the same destination folder (or some nearby subfolder).