Scrollback buffer gets slightly corrupted

Thanks for filing an issue! Please answer the questions below so I can help you.

• iTerm2 version: 3.1.3
• OS version: 10.12.6
• Attach ~/Library/Preferences/com.googlecode.iterm2.plist here (drag-drop from finder into this window) com.googlecode.iterm2.plist
• Attach a debug log, if possible. Instructions at https://iterm2.com/debuglog
• Are you reporting a performance issue or a hang? Please attach a sample. Instructions at https://gitlab.com/gnachman/iterm2/wikis/HowToSample
• Are you reporting a crash? Please attach the crash log. Instructions at https://gitlab.com/gnachman/iterm2/wikis/crash-logs Detailed steps to reproduce the problem:

1. Produce output to get data into the scrollback buffer
2. Scroll back from time to time.

What happened: What should have happened: The text should not have gotten corrupted.