Taxonomy plugin: layout error of picker in IE8

See attached screenshot. When trying to add a taxonomy term to a document, the right area of the taxonomy picker can be pushed out of its container to the bottom, if long keys are used. This problem occurs only in IE8 native (no IE9 with IE8 mode!). Reproducable in Gogreen:
-Go to taxonomies, edit one, and add some really long keys, publish.
-Open a document of type FAQ and add the really long key. Click on ok
-Again try to add some more keys, picker pops up and right area is pushed to bottom, as can be seen in screenshot.