w3c/csswg-drafts

[cssom-view] offsetTop/Left spec seems to be pretty confused when scrolling is involved

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#1,832 opened on Sep 25, 2017

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Description

Simple testcase:

<div id="parent" style="overflow:scroll; height: 100px; position: relative">
  <div id="spacer" style="height: 200px"></div>
  <div id="child" style="border: 1px solid green"></div>
</div>

Load this, scroll the scrollable div all the way down, examine "child.offsetTop". It comes back as "200" in browsers. The spec at https://drafts.csswg.org/cssom-view/#dom-htmlelement-offsettop, on the other hand says:

Return the result of subtracting the y-coordinate of the top padding edge of the first CSS layout box associated with the offsetParent of the element from the y-coordinate of the top border edge of the first CSS layout box associated with the element, relative to the initial containing block origin, ignoring any transforms that apply to the element and its ancestors.

and in this case this is definitely not 200. The spec is wrong and needs to be fixed, afaict.

Contributor guide

Tech stack
csshtml
Domain
frontend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Understanding of CSS box modelFamiliarity with CSSOM offset properties
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Review the current spec text at https://drafts.csswg.org/cssom view/#dom htmlelement offsettop and understand the scrolling behavior. Examine browser implementations (e.g., Chromium, Gecko) to see what value they return. The issue body provides a test case demonstrating the discrepancy. Propose a correction to the spec text that aligns with browser behavior. The issue may require discussion with the CSSWG.
[cssom-view] offsetTop/Left spec seems to be pretty confused when scrolling is involved · w3c/csswg-drafts#1832 | Good First Issue