When hovering over a factory building, when the preview is up, if the preview covers the factory building (such that your mouse is over both the factory building and the preview at the same time), the preview flickers. Looks like it shows the preview for 1-2 frames, then hides it, then shows it again. Let me know if you need a short video or something of this behavior or if this is actually a GUI bug in the game and I can submit it to the bugs forum. - Thanks

I know what you're talking about, there's not much I can do though since GUI elements block players from hovering over an entity. I guess I could disable the preview for a while if it flickers often enough in a short period of time, but I don't like that solution, it's rather hard to control. I'm sure the GUI redesign coming up for 0.16 will help me fix this issue.

This already been reported as an issue in the bug forum? If not I'd be happy to submit it.

The only other thing I could think of would be to make the preview a hotkey. where if you hotkey while over a factory it opens the preview until you hit the hotkey again.

It's not really a bug, it's merely a consequence of the fact/design decision/whatever that GUI elements prevent you from hovering over elements. You hover over the factory building, so the preview shows up and therefore you hover over no entity, so the preview disables again, repeat until you move your mouse away. I've made a feature request for a mouse-transparent GUI root a while ago but nothing came of it.