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.