I've seen the text "Viewing x of y | Start slideshow" being cut off partially in the lightbox that comes with TangoCMS 2.6.1. See also here: http://imageshack.us/photo/my-images/59/cutoffm.jpg for an example (content obfuscated by me). As it seems that the size of the lightbox is generated dynamically, i'm not sure if it's any use tweaking a CSS file somewhere?
It's Firefox 4.0.1 and IE 8, but it's working fine with the Carbon theme. I've modified the purity theme and both my modified version and the original purity theme seem to have this problem.
It looks like it would be the #tbOuter element that is cutting it off, and that height is set via JavaScript. I'm not quite sure what you've done to your theme to break this, but maybe you can reduce the text size or padding/margin of #tbMeta. The CSS for this is in assets/css/jquery.tangobox.css
I think I found the issue. In the carbon theme's base.css file, the line-height of the "p" element is set to 1.5em. In the purity theme, line-height is set to 1.8em. The latter seems to break the lightbox.