I've had this problem before, and it's back.
I have an elaborate folder structure for storing emails, going back maybe 20 years.
Usually it's a snap to drag one or a group of emails from my inbox to a subfolder. The past few days (on several computers running Win 7 or Win10), the focus often changes to the subfolder after I drag an email. This happens most often when the system pauses for moment when the email being moved is hovering over the subfolder.
I've always assumed that when this happens, the receiving subfolder has gotten too large, causing the pause -- so I regularly organize the subfolders so that the top-level receiving subfolders don't have too many messages, and I keep them compacted.
My usual tricks aren't working any more; I'm wondering if there might be something else causing this behavior?