Performance issue in IE9

Reproduction:

• Login using IE9 (make sure Document Mode is IE9)
• Open a Xinha editor
• Open an internal link dialog
• Click in the tree

Expect:

• Performance of the tree is OK

Actual:

• Performance of the tree is slow...

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After some investigation I found out that forcing IE9 to use Document Mode 'Internet Explorer 8 Standards' solves this issue. It's not the best fix, but it works. This can be accomplished automatically by adding <meta http-equiv="X-UA-Compatible" content="IE=8"> to the head (should be the first element in the head).