We reproduced on several PCs using Chrome and Chrome-based SRWare Iron.
The defect forces us to downgrade from 2.12.1 to 2.10.1.
Steps to reproduce:

1. zoom page to a non-default level, e.g. 125%
2. go to a pull request, then the Diff tab
3. scroll vertically with mouse or arrow keys several times
4. note the defect