Some changes occurred in HTML/CSS/JS.

cc @GuillaumeGomez, @jsha

Forgot to push the second commit updating GUI tests...

I love it! https://rustdoc.crud.net/imperio/sticky-headings/std/primitive.slice.html looks great.

I can confirm that it works as expected in Firefox on both desktop and mobile.

It's messed up when the sidebar is hidden. Otherwise, seems fine.

Kinda fixed the mess up with the sidebar button. Not perfect but sadly, we can't set an element position based on the page scroll without JS, and with JS, having a listener on the page scroll is terrible for performance. So instead, I simply make the button above the heading. It'll prevent to collapse the heading if the button is present, but I think it's ok.

@rfcbot fcp merge

Team member @GuillaumeGomez has proposed to merge this. The next step is review by the rest of the tagged team members:

• @GuillaumeGomez
• @camelid
• @jsha
• @notriddle

Concerns:

• mobile hidden persistent navigation bar (Make impl section headers sticky #133717 (comment) )
• scroll after collapse (Make impl section headers sticky #133717 (comment) )
• scroll-margin-top (Make impl section headers sticky #133717 (comment) )

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

The trouble with this sort of thing is that it's really hard to get all the interaction features right. Here's a few cases that act weird:

• If I click an item in the sidebar, it gets lost underneath the impl header.

  screen-record

  Normally, the solution to this one is scroll-margin-top, as in rustdoc mobile: fix scroll offset when jumping to internal id #93067 , but, unfortunately, we don't know how tall the impl header is:

  screen-record

• If I collapse an impl block while I'm scrolled some distance into it, I wind up at some arbitrary point in the document. I don't know how to fix this one without JS.

  screen-record

• Hidden navigation bar is still a mess on mobile:

  screen-record

@rfcbot concern scroll-margin-top

@rfcbot concern scroll after collapse

@rfcbot concern mobile hidden persistent navigation bar

(In favor of this general design, but echo notriddle's concern that this is really tricky to get right, so we should spend some time experimenting and ensuring it works in all scenarios)

☔ The latest upstream changes (presumably #136292) made this pull request unmergeable. Please resolve the merge conflicts.

I'm interested in seeing this happen to make impls more comprehensible, but I would like to recommend that this sticky header should be optional. I know that optional UI features are a cost in themselves, but I have two specific reasons that unconditional stickiness could be problematic:

• If the sticky header is tall enough, relative to the viewport (yes, I do use rustdoc on my phone) then there may not be enough remaining space to comfortably read the documentation text.
  • And how much space that is will depend not just on the viewport size but also the content and style of the documentation itself — how tall the function signatures, body text, code examples, etc. are, and how much context is needed from one part to understand another. So, a size threshold would not completely solve the problem.
• Some people find sticky headers and other content-overlapping fixed elements inherently distressing to view (I have a little bit of this myself) — it's something about losing the consistency that the entire content scrolls as a single unit within a well-defined boundary, combined with the shrinking of the effective usable viewport. Being able to avoid sticky headers should be considered an accessibility feature.