Going through the settings, turning off Community Plugins, setting the Theme to default, and disabling all CSS snippets. I’ve also opened the Sandbox vault—and it works fine there!
What is happening
When I mouseover an external link in my vault, in Live Preview nothing happens whether or not Command is held down. In Reading mode it shows a URL tooltip, not a page preview.
I did this. I turned this off. But I have no idea what I did to turn this off and make it show URLs instead of page previews, and I want to be able to turn it back on. Actually what I want is URL tooltips in Live Preview, but I’ll take the page preview.
I also cannot make it not work in the Sandbox vault, except once it showed the URL tooltip instead of the page preview, but then immediately it showed the preview again.
There is a setting to require pressing a modifier key while hovering to get the preview, tho it seems unlikely you’d have switched it by accident since it’s actually a series of switches for different contexts (Settings > Page Preview).
Oh, and it’s possible to turn off the whole feature in Settings > Core Plugins.
Well, you helped me figure out one thing: Turning the Page Preview plugin off in the sandbox vault replicated the problem. So my Page Preview plugin simply isn’t functioning at all for some reason.
This is so bizarre. Page Preview is doing absolutely nothing for some reason. And I’m pretty sure it’s due to something I did recently, because I wanted to turn it off.