Copying own comment from T190682#4086806
I thought I could edit only the top of the source code.
Indeed, if I scroll down the window and then click on the content, the page still goes back up. However, I pressed PageUp or PageDown and any other keys, and I can type anything.
If I were to continue using the mode in IE11, I would rely more on a keyboard to scroll down and then type anything. Otherwise, if I click the editing window with a mouse, the page goes back up to the beginning, so I have to use the PageDown to go back to where I left off.
Also, I found out that performing either Ctrl+X or Ctrl+C might be okay. However, once I perform the Ctrl+V, the content won't break a line.
Moreover, this is under the Vector skin. In summary, text duplication occurs under the syntax highlighting mode in old WikiEditors at IE11 while doing the above.
EDIT: Despite the issue reported at T190682, the issue with the scrolling under the mode is separate. Indeed, the textarea is editable only as long as you click only the top of the textarea and then press either PageUp or PageDown. Moreover, to edit and make changes under the CodeMirror mode, avoid clicking on the textarea while it is in the middle of a source.