From what I can see in Vista Beta 2, this is a bug in the theme itself. If I get the theme engine to draw the horizontal progressbar chunk ("progress", part 3) it draws the rather odd stretched image as seen in the screenshot. If I go for part 4 (vertical version), it repeats the chunks fine. It looks like it's simply stretching instead of repeating the image. Oops! I've have reported this bug to MS. Just to document this, for Vista the "progress" theme type has two new parts (5 and 6). These are the (horizontal and vertical, respectively) fills used in Vista instead of the XP chunked look. So, the correct visual fix is to use 5 and 6 on Vista, and 3 and 4 on pre-Vista. This also neatly avoids the Beta 2 theme bug.

This problem doesnt seem to occur in Vista RC1. Attached is a picture from Thunderbird 1.5.0.5 with Glass on.

Tested this again with the latest Vista RC and latest BonEcho nightly, and the problem no longer manifests itself (you get a "decent"--or at least "not broken" looking green progress bar now).