Chosen Solution
The screen will bounce from Safari to the desktop to mail, etc. Also, the pointer bounces around erratically and sometimes zooms in.
I suspect you have a bad trackpad. Did you have a liquid spill at some point? Or. have you cleaned your system using a spray cleaner? In either case the wetness got into the trackpad and you’ll need to replace it. To verify you can disconnect the ribbon cable from the logic board and see how a USB/wireless mouse or trackpad react. Here’s the IFIXIT guide: MacBook Pro 15" Unibody Late 2011 Trackpad Replacement and here’s the needed part: MacBook Pro 15" Unibody (Mid 2009-Mid 2012) Trackpad
