Reporting this from mediawiki.org (although I didn't manage to personally reproduce) as I also experience something similar from time to time (although not so severely).
More or less literally:
debian stretch + firefox (52.5.0 deactivated all addons) + vector skin
I just tried the new wikitext editor. The first article I tried to edit was https://de.wikipedia.org/w/index.php?title=Schwarzschild-Metrik&diff=170902272&oldid=170628851. Unfortunately the text seemed to be not in the right place, so when I selected and typed something it appeared approximately two lines below (I took some screenshots which I can upload in case they help). Also it did not tell me that I deleted the first line of the table, neither in the editor nor in the preview, I could only see it the diff-view. [...]
[...] The steps are
- enable new wikitext-modus
- enable syntax-highlighting
- go to Schwarzschild-Metrik
- click on "Quelltext bearbeiten" (edit source)
- press ctrl+f
- search for "mit dem Term"
- and you will see that the selection is already two lines above the actual text
I had a closer look at the problem again and it appears only in combination with the new syntax-highlighting beta feature. Apart from a few glitches (e.g. w:de:Wikipedia:Umfragen/Konzept für mathematische Formeln the escaped chem tag inside the template) it always worked well in the old editor and I like the more discrete and consistent choice of colors, but does not seem to work together with the new one. One can notice a wrong positioning of the cursor with respect to the text starting from about the first ref tag and the red spell-checking marks are completely off.