Why can't I scroll in the terminal?

I'm using Ubuntu 16.04 Cinnamon. After repositioning a terminal window (GNOME Terminal 3.18.3), I suddenly can't scroll up through the terminal output.

• Shift+PgUp writes 2~ on the command line instead of scrolling
• Ctrl+Shift+↑ writes A on the command line instead of scrolling
• The scrollbar on the right fills the entire vertical height of the window, indicating that there's nothing to scroll back to.

As more output is added, none of the above changes. What messed up and how do I fix it?