[layer8]

The “correct” way to do this would be to check that the surroundings of the code you’re injecting into are as expected, similar to how applying a diff patch file contains lines before/after to synchronize with the text being patched. When confronted with an incompatible change, the extension would just disable itself and not proceed with the patching. I’ve heard of MS Office extensions that function like that and work robustly.

[mst]

One of the comments on the post mentions an Explorer extension that makes the effort as well.

IMO if you're doing something that far into "don a rubber glove and root around inside somebody else's rectum" territory then the responsibility rests with the author of the crazy, not the authors of the program being fettled.

(and I'd note that I have written code that was very definitely in that territory, held myself to that standard, and having planned to do so from the start didn't find it overly onerous ... I do have a certain sympathy for people who didn't, though)