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Right click the scroll bar and choose "scroll bar options" then under "behavoior" select the 2nd option with the source overview set to "wide"

Thank you! :) I'd gone through every menu and right-clicked everywhere except the scroll-bar.

For anyone else that ends up here, I also had to click the radio button for "Map mode" too.

This. I think it used to be a productivity power tool that they since incorporated into the core editor.