focus() behavior can be used to inject or steal keystrokes — Mozilla

Google security researcher Michal Zalewski reported that focus() could be used to change a user’s cursor focus while they are typing, potentially directing their keyboard input to an unintended location. This behavior was also present across origins when content from one domain was embedded within another via an iframe. A malicious web page could use this behavior to steal keystrokes from a victim while they were typing sensitive information such as a password.