Comment 2 for bug 1542093

@LucaDC,
That may well be a valid description. But if so, it is (in my view) still inconsistent with the behavior that a user should expect from the preferences dialog.

For example, the same "steps" pane of the Global Preferences offers different step sizes for scaling an object.

If we set the "Scale" step size to be 1 px, then scaling up a 100 mm x 100 mm object (with ">") does scale that object up by 1 px in size (1 inch/90). (Scaling twice gives an object of 100.5 x 100.5 mm, just as though it had been outset by 1 px.)

And, if we set the "Scale" step size to be 1 cm, then scaling up a 100 mm x 100 mm object (with ">") does scale that object up by 1 cm in size. Scaling twice gives an object of 120 x 120 mm, just as though it had been outset by 1 cm.)

This behavior seems quite reasonable, and is consistent across different page templates (A4, letter).

If the behavior that you describe for the meaning of 'px' -- that it simply means the document's default unit -- were followed, then this would not be the case. (Or, there is a bug in the implementation of the scale step feature.)

Let us also pick an example of inset/outset with specific step size units that should be independent of the specific definition of 'px.'

Start with an A4 template, and an initial object that is 100 mm x 100 mm.
Set the Inset/Outset step size (in preferences) to 1 cm.
Use Path>Outset to increase the size of the object.

Expected behavior: should be that the resulting object (outset by 1 cm) is 120 mm x 120 mm.
Actual behavior: Object is larger, roughly 170.8 mm x 170.8 mm.

(This is outset not by 1 cm, but by 35.4 mm, the number of pixels per cm.)

Start with a letter template, and an initial object that is 100 mm x 100 mm.
Set the Inset/Outset step size (in preferences) to 1 cm.
Use Path>Outset to increase the size of the object.

Expected behavior: should be that the resulting object (outset by 1 cm) is 120 mm x 120 mm.
Actual behavior: Object is larger, roughly 280 mm x 280 mm.
(This is outset not by 1 cm, but by 90 mm, the number of pixels per inch.)

It is my opinion that this is incorrect behavior because an outset of 1 cm size should produce an outset of 1 cm, regardless of the page template used.