The synclient program is a command line tool for accessing Synaptics driver options in Linux, so that's only relevant if you're on a Linux system with a Synaptics touchpad.
On my Mac, I had this problem when using a Magic Mouse to access a Linux desktop remotely via NoMachine/NX. The Magic Mouse has no "button", but instead has a trackpad like surface that can register clicks, taps, swipes, etc. I use a third party tool called MagicPrefs for it (the key use in this context is to enable a "middle button click" effect for Linux), and I found that if I choose "Configure Scrolling" in MagicPrefs, and then disable ALL scrolling behaviors (1/2/3 finger) I no longer can trigger those "MEvent" alerts in MATLAB.
NOTE: disabling all scrolling disables the mousewheel-like scroll, and it turns out only disabling the horizontal/diagonal scrolling seems to be required to avoid the MATLAB issue.