To know what an element is offset against horizontally or vertically (Which you do with
position:relative and a
inset value) you need to know its offset parent.
The offset parent of an element is the closest ancestor that has a position other than
static, or the root element if none of the ancestors have positioning. Learn more about offset parents on MDN.
To find the offset parent of an element in Polypane:
Inspect the element in the Elements panel.
Open the Debug tab, and then scroll down to the Context section.
The Offset parent sub-section is listed there: