panel gtk tooltips remain indefinitely

I just installed Gutsy on a laptop previously running Feisty, the configuration is otherwise unchanged.

Many gtk tooltips now seem to remain on the screen for an unbounded time, until I move the mouse over them, when they disappear. Just now the "computer is now running on ac power" remained up for over a minute. I don't think this behaviour was in Feisty and I don't like it, I'm always having to wipe them off the screen. These are not the notification bubbles with [x] buttons: they're fine because they clearly require explicity dismissal. I've also seen this happen with the tooltips on panel buttons.