A long time ago, I was asked if it was possible to put the nivo shortcode into a sidebar widget.
I said yes, and developed an example on this site – in the header.
Since then I’ve had problems where the transitions weren’t displaying correctly.
In June 2013 I tried applying a fix to set max-width to none on .nivoSlider img.
Just recently, I noticed the problem on a slider being displayed in a text widget in a child theme I was creating.
And today I finally tracked down the problem.