Bug 249848 – Under certain CSS conditions, showing an element that overflows scroll re-renders fixed elements outside containers

• Status: RESOLVED
• Resolution: DUPLICATE
• Component: Layout and Rendering

[Stuart Watt]

Created attachment 464183 [details]
A small HTML+CSS+JS file. When clicking the button, the button moves

When using a position: fixed area to stick some buttons inside an overflow: auto area, showing additional content that overflows the scroll area can trigger the buttons moving, apparently outside their container by the relative offset.

A small replication file is attached.

What I've observed so far is:

1. Clicking the button, the text overflows the box, and this causes the inner position: fixed area to jump, by an amount approximately equal to the inner position: fixed area's relative offset within its parent.
2. This only happens when the height of the overflow: auto box is constrained, either by height: 100% or flex: 1 0. Without either, no issue.
3. This only happens when overflow: auto, or overflow: scroll is present. overflow: hidden and overflow: none do not have the same effect
4. It only seems to happen when there are nested position: fixed elements. Without either being fixed, no issue.
5. Safari Developer Tools still believe the elements are in their original location, i.e., you can select them in a different position from where they are rendered
6. Toggling almost any pseudo-class in the developer tools causes the elements to jump back to the correct location.

[alan]

Created attachment 464259 [details]
Test reduction

Great test case. Thanks for filing!

[alan]

This has progressed somewhere between 252435@main and 254525@main.

[alan]

Progressed at 253865@main.

[alan]

*** This bug has been marked as a duplicate of bug 222132 ***