report: fix flicker in normal case report

[yeonjuan]

Summary

Bugfix

change .lh-sticky-header's position sticky to fixed.

Related Issues/PRs

#9141

[connorjclark]

Thanks for the PR!

This needs to be sticky for devtools (#9023). Could you add a sticky declaration for .lh-sticky-header.lh-devtools, along with a comment? like:

/* The report within DevTools is placed in a container with overflow, which changes the placement of this header unless we change `position` to `fixed.` */

So we can get this nice fix for the regular report, but further work will be needed to fix this stutter in DevTools.

---

For why this is different in DevTools: the report is within a containing element. Normally, the report is the entire document, so fixed works fine in the normal case.

https://stackoverflow.com/a/48355127/2788187

If it is defined inside some container, it is positioned with respect to that container. If the container has some overflow(scroll), depending on the scroll offset it turns into position:fixed.

[yeonjuan]

@connorjclark Thanks! i changed it. :)

[connorjclark]

thanks!