Random (and unwanted) PopupPanel in GWT 2.7

I recently updated to GWT 2.7 on a project that's been using GWT since the beginning and everything was fine.  I develop on a Mac and I didn't notice any problems.

However, there's a nightly build that happens on Linux (RHEL) and checking there when I start my GWT web app a random PopupPanel appears in the far upper left as soon as I start the app.  It looks like this:

http://imgur.com/zhP8LHB

Now I do use PopupPanel (a custom extension anyway) but they don't look anything like this.  And there is nowhere in my code that adds this minimize and maximize buttons nor the resize icon that you can't see in the lower right.

And as I said, this doesn't happen in the Mac build, and it doesn't happen on the Linux build when I revert to 2.6.

Does anybody know what's going on?

Thanks!

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