Bug 189518 – Wrong getComputedStyle for 'auto' value of inset properties in fixed positioned elements

• Status: NEW
• Component: CSS

[Oriol Brufau]

Run this code: http://jsfiddle.net/msz9Lpfe/

```css
#container {
  transform: scale(1);
  border: 25px solid;
  height: 100px;
}
#fixed {
  position: fixed;
}
```
```html
<div id="container">
  <div id="fixed"></div>
</div>
```
```js
fixed.append("top: ", getComputedStyle(fixed).top);
```

If the containing block of a fixed positioned element is derived from an ancestor element, then it should be the ancestor's padding area (*).
This means that "top: auto" should resolve to "top: 0px", but getComputedStyle says "25px" instead.
Note that if you set "top: 0", the element stays at the same position as with "top: auto", and with "top: 25px" it's shifted downwards.
So the element is painted at the right position, this is just a getComputedStyle problem.
Neither Firefox nor Chromium have this problem.

(*) The spec doesn't define that for fixed positioning, but it's how browsers behave and is consistent with absolutely positioning, which is properly defined in the spec.

[Oriol Brufau]

In fact, all values are currently affected. However, in https://bugs.webkit.org/show_bug.cgi?id=188711 I plan to fix all cases except for 'auto' values.

[Ahmad Saleem]

I am able to reproduce this bug using attached JSFiddle from Comment 0 and it shows "25px" for Safari 15.6.1 and STP 152 while it shows "0px" for Chrome Canary 107 and Firefox Nightly 106. Thanks!

[Radar WebKit Bug Importer]

<rdar://problem/99533470>