fr:User:Mandariine mentioned on the French WP Village Pump that this diff of an edit she made is totally impractical since it does not show the few characters she changed. This test shows the same when editing a "p" into "ppp" in the same long line, and another test after erasing the end of the line now shows the "ppp". The "recent changes patrol" needs to detect such changes easily in order to maintain the quality of Wikipedia.
The issue of long lines in diff's has come up on Jimbo's talk page
https://en.wikipedia.org/w/index.php?title=User_talk:Jimbo_Wales&oldid=707364953 Section "We have to fix the software, nobody else will"
a particular this diff https://en.wikipedia.org/w/index.php?title=Tishma&action=historysubmit&type=revision&diff=683126701&oldid=679741880 is mentioned. A whole section of an article is a single line of text with 10K character. The diff result from this is less than satisfactory.