Locking screens properly and securely is not entirely trivial.

There is some writing from the author of xscreensaver which makes for fun reading. Displaying the desktop for a few seconds could be the least of your worries. There have been multiple security issues in the past in various screen lockers that let you completely bypass the password screen (probably all fixed today, but who knows what else could be lurking).

I won't link it directly because I think (?) his site displays something else when your Referer header is hacker news. This should provide you with an entry point, and there are multiple other linux screensaver issues over the past years linked from that post.

https://www.google.com/search?q=jwz xscreensaver "The awful thing about getting it right the first time is that nobody realizes how hard it was."

You can also search for "XScreenSaver: On Toolkit Dialogs", but I think it's linked in that blog post.