The internal diff view can get blurry on certain files when scrolling. I've noticed it on Xcode project files (project.pbxproj). I've included a screen shot. I've only noticed this issue on my external monitor.

"Backgrounding" the app and coming back to it doesn't seem to help. Moving the SourceTree window to the retina display does clear this issue up.

I have not noticed this on my retina display.