NOTE: Use the keyboard rather than the mouse to select:

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Good catch!
Actually, I'm having a hard time reproducing this behavior. The right lines always seem to be highlighted for me.
@meaghanlewis i was able to repro by selecting the lines in @jcansdale's screenshots:
Actually, I'm having a hard time reproducing this behavior. The right lines always seem to be highlighted for me.
You'll probably find this happens when using the keyboard rather than the mouse to select. The reason is that you'll tend to start (or end) the selection on the line below the selected area. If you start your selection at the bottom, this extra line will be completely invisible. If you start your selection at the top, this extra line will be obscured by the blinking caret.
I'm guessing that whoever tested this in the past used the mouse. Initially I was confused because it seemed to sometimes happen and sometimes not happen.
Does that make sense?