I first enabled both 2017 WikiEditor and Syntax Highlighting features, both in still Beta mode.
I went to an article about letter "V" and then click "Edit" or "Edit source".
Then when I opened up the editing window (or "source"), some of the grey text appear at the back of the black text:
I highlighted one word and then dragged it into another place. Then I tried undoing the action (i.e. clicked "Undo"), but then the word won't go back to its original position:
I went out the source and then back in to start over. Then I highlighted the paragraph and then dragged it into another place. So many more grey text appear at the back of the black text. I tried undoing the action, but the source somehow got worse. The paragraph went back to its original position but then disappeared when I clicked on the source. Meanwhile, it stayed like this:
I started over again. Then I highlighted just one of template codes in one line and dragged it into another place. Then I clicked "Undo", but same results as dragging a paragraph. When I clicked on the source, the template code disappears. Meanwhile, it stayed like this:
It's worse when you put a template code into the middle of another template code, making the text red. Also, some other ghost-looking (i.e. grey) text appears, especially when trying to undo the action.
There should be no overlapping text when CodeMirror syntax highlighting is enabled on the 2017 Wikitext editor on Internet Explorer.