Can't control the width of a table cell

I've spent hours trying to figure out what's wrong on the following test page : http://www.temporary-url.com/qRu63J

I just want the black tab (top right corner) to fit the width of the text. The whole thing is a table, and I set a width of 99% on the previous cell. But for a mysterious reason, it gets an unexpected width... I noticed that if I remove the big red image, it works as expected !

Thanks in advance for your help, it's driving me crazy !