how can i align objects side-by-side with 0px gap

[Boy]

Hoping that I understand the situation, one way to do this is: Clone your square (Ctrl+K) and with the Ctrl button pressed down, drag the left middle handle over to the right and when the cursor is past the first square, release the Ctrl button. Now the two identical squares are side by side without any gap.

[Acorn]

I have tried all your methods and with a n 80x80px square, I never get any gap.

The only time I can for a 1px gap is to have a Line Width of 0.5px.
If I then Snap to Objects I have the centres 80px apart.
If I display by 80px, I get an overlap.

For absolute positioning in Xara, you need to work in millipoints; you can still enter any unit but it gets immediately converted.

Acorn