This issue has been bugging me ever since I brought it up here. If you change xenforo's default login to an overlay, your saved data isn't automatically pasted in the login fields. This isn't a browser issue because I have this issue only on xenforo sites. I don't know if this issue is related to Jake's modification or to the way XenForo's overlays work. I just would like to make it work.
Most browsers probably only fill in your form fields when the page loads. If you're using the OverlayTrigger class to generate the overlay, the HTML inside the overlay doesn't actually load until you click it. When the page first loads your browser won't see any of the username or password fields from your overlay since it hasn't even loaded yet. This is just my guess, I may be wrong. So you could say it is a browser issue, and you could say it isn't. It's probably more of an issue with your implementation. For example, you could try to have the login form loaded when the page first loads, but hidden. Then when you click on login have it shown, but this would require you to write more code instead of easily using the class.