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you have WinXP's visual styles turned off the DEFAULT is for this to be ON please go to System Properties -> Advanced -> Performance Settings button to open "Performance Options" dialog turn ON "Use visual styles on windows and buttons" (very bottom of the list) you too will now see this "line" this "line" should not be present under DEFAULT settings (no Pale Moon themes and WinXP default settings)
anybody else seeing this "line" ??? (did not want the YouTube download discussion to "bury" the enquiry since YouTube is entirely pointless to me personally)
here is a nuisance i wish would get fixed don't recall how long this "line" has been in the left menu right-arrow-accessed sub-menus it's barely visible but once you do notice it then you can't "not" see it (i actually fix this via Stylem but shouldn't need to 'correct' the GUI in this manner)