The "Hide Details" feature doesn't show up correctly in the preview window

That’s interesting. Though safe mode only disables the JavaScript bit of the plugins, so it may still be one of the ones you have installed having a deeper effect. I’ve tried using Details on my test site (c81d4597a7) and here on Meta, and it works as expected, so I think it may be a conflict in your setup rather than a bug in core. What other plugins do you have on that site?