tl;dr A fix for Google Chrome 37+ which removed support for showModalDialog() and broke a lot of functionality in older releases of Sitecore and can be found on Github
11/09/2014: Sitecore have officially released patches for v6.4 to 7.0 for this bug. Glad to see that I was not a million miles off in my implementation though. Please use these patches instead: https://kb.sitecore.net/articles/581527
With the release of Google Chrome 37 we have finally lost our beloved modal dialogs created with showModalDialog(). This is used in all versions of Sitecore up to version 7.1. There is no official workaround for this yet, aside from use EnableDeprecatedWebPlatformFeatures Chrome policy, use a different browser or upgrade as stated in this knowledge base.
There was a fair amount of talk of this issue towards the end of last week, I also contributed on the main SDN thread and it seems we totally missed the issue being raised by Alex Pershteyn a few weeks ago. Well it appears those
showModalDialog() is deprecated messages in the console window actually meant something and apparently ignoring them didn’t make the problem go away 😦