Nicolas Goaziou <n.goaziou@gmail.com> writes:

> I don't understand. Are you talking about the error message? There is no
> "canonical" C-down position, so I'm a bit confused.

Put your cursor on the blank line between.  Do a C-down.  You will see
the cursor moving and also an error reported.  So, the "empty line" is a
position where a C-down operation would land you in.  It is a position
from nowhere.

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I see some issues when the point is position some where deep inside
invisible parts of the tree.

For example, visit

    "~/src/worg/org-tutorials/org-reference-guide-es.org"

Get in to a state where ALL headlines are visible but none of body text
is visible.

Position your cursor randomly, like so.

   M-: (goto-char (mod (random) (point-max)))

Then C-down.

Within let's say 10 attempts, you will see that a headline is skipped
and the cursor lands on subsequent headline.