This isn't a bug. @
cclaerhout wasn't sure, but I just fired up Internet Explorer 8 and tested my site running XMG 1.2.2 and the editor works fine. Pretty sure it's the same in IE9 and IE10 and IE11.
Indeed it doesn't work in Compatibility View (in any version of IE I tried) but that doesn't matter.
If you have Internet Explorer 8+, you should not be running a site in Compatibility View. The only reason Compatibility View exists is to provide legacy support to websites that are legacy and non-standard. As such, the browsing in Compatibility View will produce undesirable results.
It isn't an accurate form of testing site compatibility either (even if you use e.g. IE8 Standards Mode in a current IE version). It has its own bugs and compatibility issues and is not an accurate representation of how a website looks or performs.
From what I can gather, when @
Mike does testing of XenForo he does so with virtual machines actually running these browsers live. And, if we want to ensure compatibility of our own sites, we must do the same.
Personally, I have some old servers at work that run older Windows versions, and I use those for testing. Good news is, I've not found any major issues in IE8, even on a custom style.