A table with many columns
in normal view have a horizontal scroll
if you start editing in the visual editor
the right side of the column is not available
as there is no horizontal scrolling
for example https://ru.wikipedia.org/w/index.php?title=%D0%A3%D1%87%D0%B0%D1%81%D1%82%D0%BD%D0%B8%D0%BA:Kaiyr/%D0%93%D0%BB%D0%B0%D0%B2%D1%8B_%D0%BD%D0%B0%D1%86%D1%81%D1%83%D0%B1%D1%8A%D0%B5%D0%BA%D1%82%D0%BE%D0%B2_%D0%A0%D0%A4&oldid=68124375
See Also:
T76891: Cannot scroll down when editing long tables
T85542: Long tables - no page scrolling when moving up and down with arrow buttons