how can i remove this blue line? it's not available in customize menu.
That's a toolbar. I am afraid it can't be removed. You could remove the duplicate you seemingly have created.
lxp, you had me very confused because I didn't have that on mine. Took me a while to find the setting since I had no idea what I was looking for 
Settings / Location Bar / Toolbars: Show header even in single display mode
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