I've noticed that since I switched to Sierra (currently on 10.12.1) whenever I write a backtick followed by enter (or shift+enter), in many programs I get another backtick. This didn't happen in El Capitan.
I first noticed this in the Slack native app, where I could enter a code block by surrounding some lines with triple backticks, like
``` some code ```
But apparently the same happens in e.g. Chrome. As I wrote the example above, typing three backticks and pressing enter gave me four backticks instead of jumping to the next line. But if I do the same thing in Safari, it works like it used to in El Capitan.
I believe the Slack app was done using MacGap, i.e. it uses OSX's WebView, so the issue might have something to do with that as well. Then again, MacGap takes advantage of WebKit and thus I guess the problem should also occur in Safari (and perhaps not Chrome), if it trails back to WebKit.
I use a Scandinavian keyboard with the input source defined as Finnish. It seems to also contribute to the problem, as I can't reproduce it if I switch to US input source.
Does anyone know of a way to fix this – other than switching to a US input source?