Ever seen a “Connection Lost” message when using a bluetooth Apple Magic Mouse? If you’re lucky you haven’t but many people have. Some people see it far too frequently. I was seeing it roughly 2-5 times per hour! Usually it reconnected within seconds, but it was annoying.
There are all kinds of folk remedies and cures. It’s a situation where people believe in voodoo. Some people have put aluminium foil inside the mouse to enhance the battery contacts. Some people delete system files. Some move around nearby cordless phones.
A solution that seems to work for me — at least so far — is to switch to using 5GHz Wi-Fi for my regular Internet connection, rather than 2.4GHz. It seems a 2.4GHz signal interferes with Bluetooth enough to weaken the Bluetooth mouse signal in certain circumstances. (Technical note: Using 2.4GHz I was seeing Bluetooth signal strength for the mouse of around -55-65 dBm but with exclusively 5GHz it’s -45-50 dBm.)
In other words, try switching your broadband router to EXCLUSIVELY 802.11n or ac AT 5GHZ, and not dual-band, which is usually the default. How this is done varies from router to router (and some routers don’t even feature this) but if you’ve a Time Capsule or AirPort you can do it using the AirPort Utility on your Mac and clicking the Wireless tab, selecting the Wireless Options button, and selecting one of the 5GHz “only” options in the Radio Mode heading. Obviously ac is best, or n if your Time Capsule/AirPort is a little older.
Alternatively, switch to using an Ethernet connection and turn Wi-Fi off completely on your Mac.
Checking what signal range your Mac is using for Wi-Fi: Hold down Option (Alt on some keyboards) and click the Wi-Fi icon at the top right of the desktop in the menu bar. Look for the greyed-out CHANNEL entry in the list. You should see something similar to this:
Channel: 36 (5 GHZ, 40 MHz)
If you see 2.4 GHz then something’s gone wrong, and you should try again.