SliverMainAxisGroup + pinned headers overlap (no push-off) in 3.38.x, works in 3.35.x #178973

Steps to reproduce

Summary

In Flutter 3.38.x, a SliverMainAxisGroup containing nested SliverMainAxisGroups with SliverPersistentHeader(pinned: true) stops pushing the previous header out of view. The new header “dives under” the
previous one instead of replacing it. The same layout works correctly on 3.35.x and earlier.

Environment

• Flutter: 3.38.3 (stable) — bug reproduces
• Flutter: 3.35.7 (stable) — works as expected
• Dart: 3.10.1 (3.38), 3.8.x (3.35)
• Platform tested: iOS Simulator (iPhone 16 Pro)

Minimal reproduction

DartPad repro: [paste the DartPad link here]

Key structure:

• CustomScrollView
• outer SliverMainAxisGroup
  • SliverPersistentHeader(pinned: true) for Year A
  • inner SliverMainAxisGroup
    • SliverPersistentHeader(pinned: true) for Month a1
    • SliverList
    • SliverPersistentHeader(pinned: true) for Month a2
    • SliverList
• Next year section follows.

Steps to reproduce

1. Run the DartPad sample on Flutter 3.38.3 (stable).
2. Scroll so Month A1 content scrolls away and Month A2 header should replace it.
3. Observe that the incoming header overlaps/slots under the previous header instead of pushing it off.
4. Repeat on Flutter 3.35.7 — headers push off correctly.

Videos

• 3.38.3 (bug):

• 3.35.7 (correct):

Additional notes

• Tried replacing SliverPersistentHeader with PinnedHeaderSliver; the overlap still occurs on 3.38.x.
• ScrollOffsetCorrection changes and PinnedHeaderSliver fixes in 3.38 were suspected, since the regression is only between 3.35.x → 3.38.x.

Expected results

Expected behavior

When a new pinned header enters, it should push the previous pinned header off-screen (standard sticky behavior).

Actual results

Actual behavior

On 3.38.x, the new header overlaps/overdraws the previous header; it does not push it away.

Code sample

https://dartpad.dev/bc460238c9a5abd2802f9d3a1daaf4fe

Screenshots or Video

Logs

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Flutter Doctor output

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