Mouse highlights instead of single clicks.

Often when I try to single click (place a cursor between letters, or on a timeline in editing software like Pro Tools, etc.) I'm getting highlighted areas instead. I'm a longtime mac user and have used scores of macs, mice and trackpads and have never had this problem with any other computer. The problem is the same using a Magic Mouse, a wired Apple USB optical mouse, or the built-in trackpad so it's not hardware related. I've always preferred a fast "Tracking Speed" mouse preference so I thought changing that to a slower speed would help highlight less but the problem persists. To reiterate, I'm constantly highlighting areas by accident, as if between clicking and releasing the mouse is registering movement that I'm not intending. Anyone ever had this problem? Can't find anyone else having posted about it. Using a MacBook Pro 15-inch 2016 with High Sierra.