The panel of the SourceTree UI that shows the diffs of a file has a bar on the left hand side that shows the line numbers of the old version side-by-side with the line numbers of the new version. It turns out that the line numbers of the new version, which are positioned to the right, are being cropped so they can not be read.

I am not sure when this misbehavior started to happen, but it is relatively recent (it might have started after my last two upgrades). I am now using version 4.1.2 (238); just installed it today to check if that issue was solved, but it was not.