When the focus is in a translation section(editable), pressing up or down arrow will not make the page scroll. Instead the cursor will go up and down and stop and the section boundary(for example - a paragraph)
This is because the every section is individual contenteditables. Special key handling may require to get the focus to the section above or below.
Reported at https://www.mediawiki.org/wiki/Topic:T5uoxycm1mt6fku1