Thread titles being saved by my Chromebook and popping up in strange places!

[Addled]

So I was just on a gardening forum and went to post a new thread on it, only to find that when I hover over the thread title box a load of my mumsnset thread titles are offered up in a drop down box. I really dont want this to happen, my chromebook is sometimes used by other family members and I really don't need them seeing what private stuff I've thrashed out on MN! I have just completely cleared my history to no effect. Anything else I could try??

This is for the browser on a PC rather than a Chromebook but I imagine it should be the same.

allthings.how/delete-autofill-chrome/

If they're on a different user account, the suggestions won't show. You can use alt-shift-delete to delete them individually (I think? from the last time I used a chromebook. The shortcut is different on windows.)