Fix Agents composer CSS specificity

[Henning Dieterichs (hediet)]

Summary

Make Agents composer styling independent of stylesheet order by giving contextual owners enough specificity to beat generic workbench and chat rules.

The native new-session composer now consistently uses its explicit in-session toolbar overrides. This intentionally collapses the generic empty 28px bottom toolbar; the same appearance previously occurred only when stylesheets were reversed. The other composer fixtures preserve their normal-order appearance.

Cascade conflicts

Intended rule	Competing rule	Affected declarations
.new-chat-in-session .new-chat-widget-container .new-chat-bottom-container	.new-chat-widget-container .new-chat-bottom-container	max-width, padding, min-height, gap
.new-chat-in-session .new-chat-widget-container .new-chat-bottom-container .action-label	.new-chat-widget-container .new-chat-bottom-container .action-label	height, padding, font-size
.session-background-activities .session-background-activities-button .codicon[class*='codicon-']	.monaco-workbench .codicon[class*='codicon-']	font-size
.interactive-session .interactive-input-part .chat-input-container … .monaco-editor	.monaco-workbench .part.auxiliarybar > .content .monaco-editor	background-color
.agent-sessions-workbench .interactive-session .interactive-input-part .chat-input-container … .monaco-editor	the generic chat-input background rule above	background-color

Inline comments record each non-obvious specificity relationship next to the affected rule.

Validation

• Rendered 56 composer fixtures in product and fully reversed stylesheet order: 0 exact image mismatches, 0 errors (32 mismatches before the fix).
• Verified all 48 non-new-session product-order fixtures retain their previous hashes; the 8 native new-session fixtures consistently select their intended contextual overrides.
• Focused stylelint passed for all four changed stylesheets.

[Copilot]

Pull request overview

Makes Agents composer styling deterministic regardless of stylesheet order.

Changes:

• Strengthens contextual composer and toolbar selectors.
• Ensures embedded chat editors receive the correct background.
• Documents non-obvious cascade relationships.

Show a summary per file

File	Description
chat.css	Strengthens embedded chat input background rules.
sessionBackgroundActivitiesControl.css	Preserves compact activity icon sizing.
newChatInSession.css	Prioritizes in-session composer overrides.
chatView.css	Preserves Agents-specific editor backgrounds.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 1
• Review effort level: Medium

[github-actions]

Screenshot Changes

Base: 70a440a9 Current: f502fd6a

Changed (8)

sessions/chat/newInput/newChatInput/Default/Dark

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sessions/chat/newInput/newChatInput/Default/Light

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sessions/chat/newInput/newChatInput/Selection/Dark

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sessions/chat/newInput/newChatInput/Selection/Light

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sessions/chat/newInput/newChatInput/SlashCommand/Dark

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sessions/chat/newInput/newChatInput/SlashCommand/Light

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sessions/chat/newInput/newChatInput/VariableReference/Dark

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sessions/chat/newInput/newChatInput/VariableReference/Light

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1 insignificant change(s) omitted (≤20 px, Δ≤2). See CI logs for details.