MacOS – OS X Mavericks scrollbars appear even without external mouse

I've had the same issue, starting with a late release of Snow Leopard, and now with Lion. The mouse works fine for a while and then it gets really laggy. Turning the mouse off (disconnecting bluetooth with the computer) and turning it back on fixes the problem, but it has happened at least a dozen times in the past week or so.

I've reported it as a bug to Apple, but it is hard to reproduce. I also have a MacBook Pro with an external monitor. I'm not sure if that is just coincidence, though.

Are you using a trackpad by any chance? I too thought the way the Dock popped up on different monitors was completely arbitrary or broken, until I found out how it actually works:

• If you move the cursor to the bottom edge of either screen and keep your finger on the trackpad, the dock will pop up right away
• If you flick the cursor to the bottom edge of either screen (removing your finger from the trackpad as you finish the movement), the dock will not pop up

This is designed like this intentionally so that as you're mousing around in full-screen apps, you don't inadvertently trigger the dock when you use controls at the bottom of the full-screen window.

It actually works really well, once you know how it works ;-)