Comment 2 for bug 1669360

Like a charm!
Thank you for the clue.

P.S.
About "Caution" part: you put me in stunt for a while.
It was difficult for me to understand,
but I did series of experiments and now I see:

Let's have square [10 x 10 mm] in document with "mm" units;
(remark: let's assign X=0; Y=0 )

case 1:
When we change units let's say from "mm" to "px"
Inkscape preserves created object dimensions "as is" so our 10mm becomes 37.795px.
Our square now [37.795 x 37.795 px]
and it hasn't change its location.
"Scale factor" appears to make this kind of conversion,
in our case it is auto-become "0.26458" ("Scale x" in Scale section of Document Properties (Shift+Ctrl+D) as well as "Scale y" (!) )

case 2:
Alternatively changing units from "mm" to "px" and manually override "Scale factor" ("Scale x") to "1" will give us objects with the dimensions of the same values as we've entered creating them but for other units ("px")
"Physically" objects are scaled to suite new chosen units.
Me personally would prefer case 2 much more (because I'm addicted AutoCAD user (my main job), and I used for such behavior for ages)))
But here I found one unpleasant "stepping stone":
Inkscape keep "Scale y" dimmed, so then my everything "jumps" in ordinates after unit changing manipulation!
Our square become [10 x 10 px]
but coordinates now: X=0; Y=825.520 (Why?! I didn't solve this since it is "0" initially!?)

P.P.S.
Please help me:
Personally me can live with this ))) but ...
May it be considered as bug or am I miss something?

I can feel phrase mentioned near "Scale":

"While SVG allows non-uniform scaling it is
recommended to use only uniform scaling in
Inkscape. To set a non-uniform scaling, set the
'viewBox' directly."

should explain everything (why "scale y" is dimmed), but if I understand the meaning proper it causes exactly what it should prevent!