My experience, having explored this issue with 4 Magic Mice and four macs (First intel Mac Pro (1,1), latest Mac Pro (6,1), First MacBook Pro with Retina, 15" (10,1) and older MacBook Pro (4,1)), is as follows:
- I never had a problem until I changed the batteries in the mouse from the ones Apple supplied
- While I had some disconnections on all systems, they were much more common using any of the mice with the older Macs, particularly the old Mac Pro)
- I could improve the situation by wrapping the batteries in paper or adding foil at the contacts, or both, but that didn't completely solve the problem
- All four of my mice seemed to behave in very similar ways
- The problem only occurs with some brands of batteries.
When I started noting what batteries I was using, I found that the problem only arose using particular brands of battery, primarily Duracell. If I stick to either Apple Rechargables or ordinary Energizer AA batteries, I never get a disconnection.
I think the fundamental problem is that some batteries are smaller than Apple designed for, and those ones lead to intermediate disconnections. But their bluetooth implementations shipped in Macs have improved over time and the newer ones are less sensitive to very short bluetooth interruptions and don't lead to a loss of connection.
The bottom line is, whatever the various underlying problems are, finding the right kind of battery and sticking to it has eliminated the problem completely for every mouse I've tried with in combination with every Mac I've paired them with.