Some users have run into an issue when hosting ASP.NET Ajax applications in a frame or iframe that's in a different domain from the top-level window. If you try to do that and browse to the page using IE, you'll receive an "access denied" error client-side any time a DOM event is raised in the frame. The code that's responsible for that is Sys.UI.getLocation. This is a tricky piece of code that determines the pixel coordinates of a DOM element relative to the top-left corner of the page, so that an absolutely positioned child of the body using these coordinates would exactly cover the element you measured. This is useful for drag & drop, popup scenarios like auto-complete and when an event is raised to get mouse coordinates relative to the event source. This is this last piece that explains the problem. The code in this method is different for each of the browsers that we support because each one of them has its own behavior that cannot be determined as we usually do by dynamically looking at capabilities. You just have to know for example that browser X is not counting an element's scroll position if it's absolutely positioned and a direct child of body (names changed to protect the guilty). That's the kind of problem we had to work around. Luckily, IE has two very handy methods to retrieve this kind of coordinates that enables us to bypass completely a number of bugs that we just couldn't efficiently work around: getClientRects, which gets all rectangles the element occupies on the page, and getBoundingClientRect, which returns a single rectangle that bounds the whole element. In the method that we shipped, we've been using getClientRects and getting the first rectangle because we wanted to have consistent behavior across browsers even if the element is a wrapping element such as a span: in this case, the top-left corner of the element is the top-left corner of the first bounding rectangle, which is different from the top-left corner of the global bounding rectangle:
I'm very proud to announce that ASP.NET Ajax 1.0 shipped this morning. This is the result of all the work we've done since the first CTP and of all the very valuable feedback from our early adopters. I can't stress this enough: this product is probably one of the Microsoft products that incorporated the most user feedback. It is also to my knowledge the first time that Microsoft releases the full source code for a supported product (yes, we have full 24x7 support for ten years).