Try to copy https://en.wikipedia.beta.wmflabs.org/wiki/CodeMirror/src?veaction=editsource to https://en.wikipedia.beta.wmflabs.org/wiki/CodeMirror?veaction=editsource (for example to beginning of text). Then try to change something after. Changes are not visible on clicked (with caret) line! Tried on Firefox but try to zoom* page if you can not reproduce.
- - I think the cause is synchronization with line heights.