The showing of ComboDropList items

[iversion]

I add some Chinese characters as item to ComboDropList. Then I found that some characters can't be shown in full height, that is ,the top of charaters is covered by the background image of the ComboDropList.
It seems to have something to do with the resolution. If the ogre window resolution is 1280*1024, the space between the list box items is wide and every item shown clearly. But if the resolution is 1024*768, then the situation I mention above will happen.
Can anybody explain why?