tom_5200 reported on IRC that IE, though it suspends events in all frames, only suspends loading in the calling frame. So, if a subframe calls showModalDialog, loads in the main frame should continue. I haven't tested this.
Created attachment 12957 [details] Contains a testcase for the bug Test case: 1) Open "modaltest.html" in the browser. 2) Click the "modal test" button. 3) In the modal dialog, click the "async" button. Expected behavior: The request should be asynchronously executed immediately in the context of the main window, and an alert box should appear with the text "Asynchronous AJAX test ok". This is the behavior in IE. Actual behavior: The request is not executed until the modal dialog is closed. Despite being performing the delayed request, no "onreadystatechanged" events are fired to the browser.
showModalDialog support has been removed from upstream. https://github.com/WebKit/WebKit/commit/c4bb4d1af89e1ef5f57ef1b1cf7b65ce62c30ba5 Hence, I am marking this bug as "RESOLVED WONTFIX". Thanks!