Since updating to Symfony 3.3.4 from 3.3.3 (or could have been 3.3.2), there has been an issue with the dump hover in the dev toolbar. I have updated to 3.3.5, but the issue is still there.

What happens, is when you hover over the dump icon in the dev toolbar (the target), it flashes on and off as you move your mouse over it (as though the hover is on the path, not the entire button), and you can not move your mouse over the popup box (with the variables in it) as it disappears before you can get there (for example, to click an arrow to open an array).

NOTE: I can not be 100% sure of when this stopped (3.3.3 or 3.3.4) working, except that it worked in a default 3.3.2. This is because when I tried to download a 3.3.3 project, the default was broken! This means that since version 3.3.3, someone has modified code in the 3.3.3 project, so I can't tell if this has caused the var dumper issue in it as well. (the error with the default 3.3.3 is Attempted to call an undefined method named "getParameter" of class "AppBundle\Controller\DefaultController".)