Google Chrome 67.0.3383.0 (Official Build) dev (64-bit)
Google_Samus.6300.276.0
Chrome OS with flag enabled: #enable-experimental-accessibility-features

# Enable STS
# Go to Customize and Control Chrome menu and set the browser zoom to either smaller than 100% or larger than 100% 
# Invoke the feature using the search button + drag the focus ring around text. I used this page: https://en.wikipedia.org/wiki/Tardigrade 
Expected: only selected text read
Actual: STS reads and attempts to highlight text that comes before and after the selection, even if that selection is outside the visible frame.  

This does not happen with selecting text first and then using search + s. 

I could repro this from 33% up to 75% and then 125% all the way up to 500%. I couldn't get it to repro at 25% because the text was just too small for me to see what was happening. I also couldn't repro at values around 100% or at 100%.