I would like to know how to disable the automatic expansion of folders when my mouse hovers over them in the navigation pane. I've searched all over the folder options and have searched online, but no one has an answer.
I've not seen this behavior on any W10 computers; and I have 10 of them. Just a quick check on the 3 most current W10 versions (v1511-2015),(v1607-2016), and (v1703-2017) on 3 different computers does not exhibit this behavior either. I even went into Folder Options in File Explorer and tried to make the folders open on auto-hover; no go.
I suspect no one has the answer, as it's not a normal function in W10; therefore there is no way to turn it off. I would suspect a virus/malware. Try scanning with whatever AV program you have (Norton, Avast, Windows Defender) and scan/remove all found viruses. Next, visit malwarebytes.org and download their free antispyware program and scan/remove all spyware viruses. Reboot and retest your computer; fire up File Explorer. If the problem vanishes, you had a virus and it's been removed and you're set to go!