Chrome Version 67.0.3396.87 (Official Build) (64-bit)
MacOS 10.13.5 (17F77)
This is a weird one, and I haven't figured out how to reproduce it, but opening this task to record my observations.
I got into a state where as I was trying to select text by dragging the mouse over it, I was getting text selected one line below where my mouse cursor was.. This was entirely reproducible within that editing session, but I haven't yet been able to reproduce it in another window.
Note: the attached screenshot was taken with Grab and timed screen capture. I'm not sure how faithfully that reproduces the cursor location, but the image is a reasonable representation of what was happening.
If I moved the cursor up and down a few pixels, it acted as if the cursor hotspot was at the very bottom of the drawn cursor icon. Normally, for the insertion I-bar, the hotspot is the vertical center of the icon.