From [[Wikipedia:VisualEditor/Feedback#Editing_Defaultsort]]:
The Page Settings box displays the article title as a default "Defaultsort". But it doesn't seem possible to edit it: typing any letters removes the whole existing text, while trying to delete a few characters has no effect. Where the sort key is not identical to the article title it's usually a variation of it - delete a leading "The ", or move the given name from start to end. It's a waste to have to retype it all: we ought to be able to edit that box.
This is because the default defaultsort value (article title) is implemented as "placeholder" text. Instead, it should be normal editable text.
Upon closing the page settings dialog, the logic should be, "if defaultsort != article title, add a {{DEFAULTSORT:}} magic word, else remove existing {{DEFAULTSORT:}} if present".