Steps to recreate:
OSX 10.11.6
Firefox: 51.0.1
{F5742661}
1. Open https://en.wikipedia.beta.wmflabs.org/wiki/Dog
2. Hover over **carnivore**
3. Open in new tab clicking (Ctrl+click) the link
4. Move mouse and go to other new tab, see hovercard disappear
5. Close new Carnivore tab, when the **Dog** tab gets re-focused...
==Expected==
6. There's no preview
==Actual==
6. The **carnivore** hovercard will re-appear, and won't close until clicking somewhere or hovering the preview
~~Note: not observed in Chrome or Safari browsers~~
Using the ctrl+link method it also happens on the following OS and browsers:
* Windows 10 - Edge & IE 11 & Chrome 56
* Windows 8.1 - IE11 & Chrome 56
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Seems like this is related to the focus event on the link being re-triggered when the tab is re-focused...