(Split from bug 45051.)
I have no idea what this is, but it's probably disabled entirely and unknown
of for a reason.
Quoting from bug 45051:
By its name, it looks like this could be code to automatically
expand the search box when clicked or when the search sting
exceeds a certain length. Just guessing, though.
When focused. It's important for search usability for the search
input to be large enough to accommodate practically anything the
user would input into there.