Messages App Moves Around on Desktop

After installing Yosemite, I've noticed that the Messages app likes to move around on the screen when I click on it.

This is, as you can imagine, really, really annoying. What's even more annoying is not being able to find a scrap of anything online that addresses this. Surely, someone else must be experiencing a similar issue, right? Or am I really patient zero?

Anyway, I'm using an early 2011 MacBook Pro.

What usually happens is I'll click on the app to make it active, then click in the field to compose a message. When I do this, the app moves so that the top edge of it becomes located where I clicked. I'll move it back to where I like it located, click in the "compose message" box, and I'm able to type the message and send it properly. However, when I click away from the app (to Mail, Safari, the Desktop or wherever), the Messages app moves again. I move it back to where I want it and this whole process starts over.

I've tried closing and reopening the Messages app, restarting and repairing permissions.

I haven't witnessed this behavior with any other applications.

Thanks for your help!