Introduced and fixed issues report
This report allows you to get a better understanding of how a particular change has affected overall code quality.
Upsource computes the difference between the previous and the current revisions of the file and shows you details of the issues (errors and warnings) that were introduced or fixed in the changes you're looking at:
A separate panel at the bottom of the page lists introduced and fixed issues from all files included in the revision(s):
The Introduced and fixed issues report is enabled by default. You can turn it off in your personal user settings.