fix(chat): action buttons hidden under next turn sticky header

[bashrusakh]

Summary

Fixes openchamber#2095 — action buttons (copy, edit, etc.) at the bottom of assistant messages were hidden/overlapped by the next turn's sticky user header due to a CSS stacking context conflict.

Root cause

The assistant block used relative z-0 which created a stacking context at z-index 0. The next turn's sticky user header used sticky top-0 z-20 creating a stacking context at z-index 20. Both were in the same parent stacking context (ChatContainer's relative z-0), so z-20 painted on top of z-0 — the next turn's sticky header covered the previous turn's action buttons.

ChatContainer (relative z-0) ← root stacking context
├── Turn N section (relative w-full — no stacking context)
│   ├── sticky header (sticky z-20) ← z-index 20
│   └── TurnAssistantBlock (relative z-0) ← z-index 0
│       └── action buttons ← trapped inside z-0
├── Turn N+1 section (relative w-full — no stacking context)
│   ├── sticky header (sticky z-20) ← z-index 20, paints over z-0
│   └── TurnAssistantBlock (relative z-0) ← z-index 0

Fix

Three minimal CSS-class changes across 3 files — no logic/behavior change:

1. TurnItem.tsx — removed redundant relative from the sticky header (sticky is already a positioned element, so relative was redundant and confusing per issue report)
2. TurnAssistantBlock.tsx — removed z-0 so the assistant block's children are no longer isolated at z-index 0 and can participate in the parent stacking context
3. MessageBody.tsx — added relative z-30 to all 3 action button containers (inline actions ~L1743, standalone actions ~L2064, turn footer ~L2072) so they paint above the sticky header's z-20

Files changed

File	Change
packages/ui/src/components/chat/components/TurnItem.tsx	removed redundant relative from sticky header
packages/ui/src/components/chat/components/TurnAssistantBlock.tsx	removed z-0 from wrapper div
packages/ui/src/components/chat/message/MessageBody.tsx	added relative z-30 to 3 action button containers

Validation

Check	Result
bun run --cwd packages/ui type-check	✅ pass
bun run --cwd packages/ui lint	✅ pass
OCR code review (glm-5.2)	✅ 0 comments — Looks good to me

Notes

• The bot comment on Action buttons hidden under overlapping messages openchamber/openchamber#2095 mentioned transform: translateY(...) in MessageList.tsx as part of the root cause. Verified that MessageList.tsx actually uses paddingTop (not transform) — the code explicitly avoids transform to prevent breaking sticky behavior. The stacking conflict is real, but the cause is the z-0/z-20 interaction in the parent context, not a transform.
• Related issue: Chat message visual order reversed - assistant appears above user openchamber/openchamber#2094 (visual order reversed) — same stacking-context family of problems.

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[bashrusakh]

Closing — created in wrong repo (fork instead of upstream). Will reopen against openchamber/openchamber.