This may be a case of "Bad things happen when VisualEditor Converter and Parsoid disagree with each other".
Initial symptom: The font size changed in part of the editing window. See a screenshot at https://upload.wikimedia.org/wikipedia/commons/e/ef/Problem_WP_Editor.jpg
Then the editor seemed to crash.
Reported in great detail at https://de.wikipedia.org/w/index.php?title=Wikipedia:Fragen_zur_Wikipedia&oldid=167554917#R.C3.A4tselhaftes_Verhalten_des_2017-Quelltext-Editors (complete with console output) by @XanonymusX, who can help answer questions. He usually uses SRWare Iron on Win 10 Pro, but also tested this in &safemode=1 in Chrome and Microsoft Edge.