Constant DOC_SIZE_JS 

pub const DOC_SIZE_JS: &str = "JSON.stringify({width: Math.max(document.documentElement.scrollWidth, document.body ? document.body.scrollWidth : 0), height: Math.max(document.documentElement.scrollHeight, document.body ? document.body.scrollHeight : 0)})";

Expression yielding the document’s scroll size as JSON, for full-page screenshots on BiDi.