Function history_event 

pub fn history_event(method: &str, event: &Value, main: Option<&str>) -> bool

Whether a page event asks the bar’s history again. Only the main frame’s navigation moves it: a subframe’s frameNavigated fires too, and a read per ad iframe is nobody’s time. A same-document entry moves no frame and fires no load, so it names itself - and names the frame it happened in, which is why main is checked for it too.