What causes this to happen? (the mouse is not being moved or clicked)
I suspect that the :hover CSS style results in the object having a different size (possibly margin), which causes the :hover CSS style to cease to be applied. This returns the object to its original dimensions, and the :hover CSS style is applied by the browser once more.
The browser can only keep up with this at a certain rate and you see visible flickering.
It's an edge condition.
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