unique constraint violation on multiple-rows update

Dear all,

I am trying to update (business logic list) position
information of rows in a table. There is a unique constraint
on those positions. The relative order of rows needs to be
preserved but list positions need not be gapless.

The idea was to move out of the way any existing conflicting
row(s) by incrementing the list position. I tried to use a
CTE that returns rows in DESCending order and use that to
drive an UPDATE, like so:

    WITH cte AS (
        SELECT pk, list_position
        FROM clin.export_item
        WHERE
            list_position >= _target_position
        ORDER BY
            list_position DESC
    )
    UPDATE clin.export_item SET
        list_position = cte.list_position + 1
    FROM cte
    WHERE
        clin.export_item.pk = cte.pk;

Running that does violate the (non-deferred) UNIQUE
constraint on the table column, however.

The Dept of Second Thoughts tells me that that UPDATE does
not care or even know about the CTE order and just updates
rows in whichever order it sees fit.

Is there a correct or better SQL idiom to use for the use case ?

This does run as part of a plpgsl function so I can rewrite
as a loop but I wonder whether I should be able achieve
the objective with a single UPDATE.

Many thanks,
Karsten
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