Bug 176384 – Fixed-position block within fixed-position, overflow-scrolling block is unexpectedly clipped

• Status: RESOLVED
• Resolution: CONFIGURATION CHANGED
• Component: CSS

[Evan Chaney]

Created attachment 319917 [details]
Initial test case demonstrating the described issue

OVERVIEW

- Given a fixed-position block (A) with `overflow: scroll`
- And a second, fixed-position block (B) within it
- When block A is positioned to fill the entire right half of the viewport
- And block B is positioned to occupy the center of the viewport
- Then the left half of block B is unexpectedly clipped by block A

STEPS TO REPRODUCE

- Download attached test case (test-case-01.html).
- Open test case in Safari.

EXPECTED RESULTS

- Modal dialog is fully visible.

ACTUAL RESULTS

- Left half of modal dialog is clipped.

BROWSERS TESTED

- Safari 10.1.2, macOS 10.12.6: issue occurs
- Safari nightly 10.1.2 (12603.3.8, r221610), macOS 10.12.6: issue occurs
- Firefox 55.0.3, macOS 10.12.6: no trouble found
- Chrome 60.0.3112.113, macOS 10.12.6: no trouble found

[Evan Chaney]

Although nested-fixed-position blocks are a somewhat odd construction as best I can tell this case does appear to be a valid construction/expectation.

The CSS 2.1 spec. establishes blocks in normal flow with overflow other than “visible” establish a new formatting context:

https://www.w3.org/TR/CSS2/visuren.html#normal-flow

However, “CSS Positioned Layout Module Level 3” establishes absolutely-positioned blocks are removed from normal flow:

https://www.w3.org/TR/2016/WD-css-position-3-20160517/#pos-sch

It also establishes fixed-positioned blocks are included when the spec. references absolutely-positioned elements:

https://www.w3.org/TR/2016/WD-css-position-3-20160517/#abs-pos

Given these rules it seems a fixed-position block should not be subject to the formatting rules of a containing parent.

[Brent Fulgham]

Safari and Firefox agree on rendering for this test case. I don't believe there is any remaining compatibility issue.

Chrome seems to lack select controls, but that doesn't seem related to this clipping complaint.