I have a page like this:
<Grid>
<StackPanel HorizontalAlignment="Center" VerticalAlignment="Center">
here are contents
they are forever absolutely in the center of the screen
no matter of the resolution/size
</StackPanel>
</Grid>
Everything is working fine. But now I want to add a back button in the top-left corner.

And I don’t want to split the grid into 2 columns and 2 rows like this:

the contents are no longer absolutely centered, even we can split the grid by percent, because the contents are sometimes very wide and sometimes very narrow.
I want to know how can I keep the contents horizontal/vertical aligned “Center”, and then set the back button’s position relatively to the content.
slightly hacky ansewer
You might be able to achieve by positioning your stackpanel in the center, and then set a negative left margin the width of the button to shift everything left by the required amount…