sessions: back the checkpoint and fork rows over a chat background

[TylerLeonhardt]

The Restore Checkpoint, Start Over and fork controls in the conversation are transparent at rest — only :hover gives them a fill (chat.css:4379, chat.css:4410). Neither row sits inside the opaque .request-hover; all three are siblings on rowContainer (chatListRenderer.ts:1084/1100/1172). So over a chat background they read straight on the wallpaper.

This is the same gap #334655 closed for the new-session composer, one level up in the transcript.

What changed

One rule in the sessions layer, scoped to .has-chat-background. It backs the toolbar and its sibling text with one opaque plate rather than plating each control — the move chat.css:4574-4580 already makes for the editor-instance restore row.

Plating the container instead of the controls buys three things:

• The controls stay transparent, so their existing hover fill composites over the plate. No specificity fight with chat.css:4410.
• The .confirming Restore Checkpoint pill still owns its own surface and keeps its background-clip: text gradient (chat.css:4422-4437). No exception needed.
• No permanent border where today only hover draws one. The rows keep their quiet divider tier.

.checkpoint-dot-separator is the one selector beyond the editor-instance precedent: it sits between the label and the toolbar on the restore row, so leaving it out would strand it on the wallpaper between two plates.

Appearance without a chat background is untouched — nothing in workbench chat.css changes.

Verification

• ./scripts/test.sh --grep "Sessions - Chat View" — 31 passing. The new case builds both rows under a backed and a plain .part.sessionspart and snapshots the four fills against rgba(0, 0, 0, 0).
• Checked live in an Agents window over the codicon wallpaper. .checkpoint-container .monaco-toolbar computes rgb(25, 26, 27) (--session-view-background); "Restore Checkpoint · ⑂" reads as one continuous band, and the hover fill still lands on top.
• .checkpoint-restore-container has gap: 4px, so its three plates leave two 4px seams. They're invisible in practice — the row reads as one band. If a busier image ever exposes them, the fix is a box-shadow bleed on the plated elements, which closes the gaps without touching layout. Not adding it preemptively.

🤖 Generated with Claude Code

[Copilot]

Copilot review overview

🟢 Approval recommended

The focused styling change matches the stated behavior and includes appropriate regression coverage.

Review tier: Balanced
Findings: None

What changed in this PR

Adds opaque backing surfaces to checkpoint and fork controls when Sessions uses a chat background.

Changes:

• Applies the session background beneath checkpoint controls and restore-row text.
• Adds regression coverage for background and plain-session rendering.

File	Description
chatView.css	Adds chat-background-specific control backing.
chatView.test.ts	Verifies conditional opaque fills.

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