How to limit iTerm2 text selection to a single tmux pane

I have a late 2013 MacBook Pro running iTerm2. I don't have tmux installed on it, but I use tmux on the remote Linux host I frequently connect to via ssh. When tmux is displaying a vertical split and I use the trackpad to select text, the selection is automatically limited to just one pane. That's exactly how I want it.

I have recently begun using a 2018 MacBook Pro. Using the same iTerm2 version (3.2.9), and the same remote tmux session, text selection spans the entire window width instead of staying in a single pane.

What setting controls that behavior?

I'm aware I can hold Command+Option to select a block of text, but that's not what I'm after. I'm looking to select, for example, a sentence that starts mid-paragraph and spans multiple lines. Block selection doesn't allow that.