Follow-up from issue #2990 (design locked).

Locked decisions (from issue #2990)

1. Scope: Pull request File Changes only (other PR tabs already sticky).
2. Behavior: Both bars sticky and stacked — primary PR tabs stay at the top; Filter by path / Diff options stay immediately under them with no overlap.
3. Out of scope: Build / Issue parity smoke beyond PR File Changes.

Problem

.sticky-tabs-row is sticky globally, but base.css intentionally disables it when a revision diff is present:

html:has(.revision-diff) .sticky-tabs-row {
  position: static;
}
html:has(.revision-diff) .sticky-tabs-row.is-docked>.more-info-dock {
  display: none;
}

PR File Changes embeds RevisionDiffPanel (.revision-diff). That exception lets .revision-diff > .head own top: 0 alone (from issue #2954 / issue #2969). Product now wants both bars sticky and stacked.

Do not “fix” this by adding Bootstrap .position-sticky / !important on the tab row — that only overrides the exception and leaves both bars at top: 0.

Goal

On PR File Changes, after scrolling:

1. Primary tab labels (Activities / File Changes / …) + docked more-info stay pinned at top: 0.
2. Revision-diff toolbar (Filter by path, Diff options, …) stays pinned immediately under that row.
3. Neither bar covers the other.

Implementation plan

1. Re-enable sticky tabs when .revision-diff is present

In base.css:

• Remove (or reverse) html:has(.revision-diff) .sticky-tabs-row { position: static; }.
• Remove (or reverse) html:has(.revision-diff) .sticky-tabs-row.is-docked>.more-info-dock { display: none; } so the more-info dock can appear again while Changes is sticky.
• Review the older sibling rule html:has(.revision-diff) .nav-tabs-line.nav-tabs-sticky { position: static; } and keep it consistent (legacy marker vs .sticky-tabs-row).

2. Offset .revision-diff > .head under the sticky tab row

In revision-diff.css, .revision-diff > .head is currently:

.revision-diff>.head {
  position: sticky;
  top: 0;
  z-index: 10;
  height: 64px;
  …
}

Set top to the live height of .sticky-tabs-row (not a hard-coded guess only), so the path-filter bar sits flush under the tabs.

Because the tab row height can change (wrap, docked more-info, font zoom), sync via JS (resize / ResizeObserver on the sticky row), e.g. from revision-diff.js or PR detail JS:

• head.style.top = stickyRow.offsetHeight + "px" (or equivalent).
• Keep z-index of .sticky-tabs-row (990) above .revision-diff > .head (10) so stacking order matches visual order.

3. Fix scroll anchoring for in-diff jumps

revision-diff.js currently sets:

$(".revision-diff>.body li.diff, …")
  .css("scroll-margin-top", $(".revision-diff>.head").outerHeight() + "px");

Include sticky-tab-row height as well (tabs + head), otherwise “jump to file” / active navigation can land under the stacked sticky bars.

4. Manual verification (PR detail only)

On a PR with a non-trivial File Changes view:

•  Deep scroll: tab labels stay at top; Filter by path / Diff options stay directly under them with no overlap.
•  Tab wrap / narrow width: head top still matches sticky-row height.
•  More-info dock appears when header more-info scrolls away; side-info open/close still hides triggers correctly.
•  Activities / Commits / Builds / Code Comments unchanged (still sticky as today).
•  In-diff navigation / path jumps clear both sticky bars (scroll-margin-top).

Non-goals

• Bootstrap .position-sticky / !important as the long-term fix.
• Changing sticky behavior on Issue / Build detail pages (issue #2990 Q3=A).