I use plugin for that, it walks through all elements on page (createTreeWalker) and detect actual style position (getComputedStyle) and if it is "fixed" or "sticky" it changes display to "none". So far it worked with every bar, even "use app" button. Then as a bonus I remove all fonts, change color to black on white background, and turn full screen mode on.
pastebin.com/6Q8WT9WJ