Google Chrome	67.0.3363.0 (Official Build) canary (64-bit)
Firmware Version Google_Samus.6300.276.0

For now, go to Chrome://flags > Enable Experimental accessibility features to get the latest version of STS. In the future, this step won't be needed.

Steps to repro:
# Prepare an area of text with multiple groupings of text such as a Gmail message where there is a series of forwards or replies going down the page. 
# Invoke STS by holding down search and dragging ring across page including several sections
Expected: Selection focus highlight remains the same size as you put it during selection
Actual: Selection focus highlight will grow and shrink as different text areas are read. For example, it will become smaller around the address/timestamp info and then will grow larger than the entire message and highlight the entire frame instead of just the text selected. 

At times, the focus highlight will be around the top of an area of text while STS is reading below it on a screen. The word highlight continues highlighting every individual word as its being read even when it is outside the focus highlight because the highlight has shrunken from the original selection and is "stuck" to the top of the selection. 

I don't currently have the ability to take screen capture video on this Chromebook so I can repro in person for a dev if requested.