In a nutshell, it looks like an upcoming change to add 'text substitution support' to Chrome on MacOS might break code copy/pasting code from websites. The change means things like quotes and ellipsis will be substituted for non-ASCII characters, so code containing the affected characters won't be valid when copy/pasted. According to the Chromium ticket [0] it'll land in a few weeks.
It appears the issue currently affects StackOverflow. There's a question on their meta site [1] with a good description of the problem, that will probably also serve as a good place to look for potential solutions.
[0] https://bugs.chromium.org/p/chromium/issues/detail?id=42434#c134
[1] https://meta.stackoverflow.com/questions/386985