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+---
+module: frontend/chat
+date: 2026-07-03
+problem_type: design_pattern
+component: frontend_stimulus
+severity: medium
+applies_when:
+  - "Implementing message bubble styling for Vue chat components"
+  - "Deciding between CSS :empty selector and JS computed for empty-content detection"
+  - "Excluding card-bearing message types from bubble chrome"
+  - "Making Vue computed properties unit-testable without @vue/test-utils"
+tags:
+  - vue
+  - css
+  - message-bubble
+  - testing
+  - computed-properties
+  - design-pattern
+related_components:
+  - assistant
+  - testing_framework
+---
+
+# Message Bubble Empty-Content Detection & Card-Type Exclusion Pattern
+
+## Context
+
+When implementing Scheme B (neutral grayscale) assistant message bubbles in the
+chat UI, three interacting concerns surfaced that the naive CSS-only approach
+could not solve:
+
+1. **Empty assistant bubbles**: Pre-stream and tool-call-only messages render
+   an empty bubble rectangle — visually broken.
+2. **Card-bearing double chrome**: Assistant messages rendered as dedicated
+   cards (ErrorCard, BoardConclusionCard, etc.) already carry their own
+   background/border/padding. Wrapping them in an assistant bubble produces
+   double chrome.
+3. **Testability**: Project convention uses pure-TS vitest tests
+   (`tests/unit/**/*.test.ts`) without `@vue/test-utils`. Computed properties
+   embedded in Vue SFCs are not directly unit-testable.
+
+## Guidance
+
+### 1. Never use `:empty` CSS selector for Vue component emptiness
+
+Vue components always render a root element (even `<template><div></div></template>`),
+so `:empty` **never matches**. Use a JS computed property that checks the
+underlying data fields instead.
+
+```vue
+<!-- MessageShell.vue -->
+<template>
+  <div :class="[..., { 'message-shell--empty': isEmpty }]">
+    <slot />
+  </div>
+</template>
+
+<script setup lang="ts">
+interface Props {
+  isEmpty?: boolean  // computed by parent, passed as prop
+}
+</script>
+
+<style scoped>
+.message-shell--assistant.message-shell--empty .message-shell__content {
+  background: transparent;
+  border: none;
+  padding: 0;
+}
+</style>
+```
+
+Parent computes emptiness from message data:
+
+```ts
+// ChatMessage.vue
+const isBubbleEmpty = computed(() => isAssistantBubbleEmpty(props.message))
+```
+
+### 2. Exclude card-bearing types via `messageType` prop, not `:has()` selector
+
+Maintain a `Set<string>` of card-bearing message types and check membership via
+a computed. Pass `messageType` as a prop rather than using `:has()` CSS selector
+(better browser support, testable, no CSS selector complexity).
+
+```ts
+// bubbleUtils.ts (pure function — testable)
+const CARD_BEARING_TYPES = new Set<string>([
+  'board_conclusion', 'team_plan', 'debate_started', 'debate_argument',
+  'debate_summary', 'debate_resolved', 'collaboration_graph',
+  'review_result', 'risk_flagged', 'error',
+])
+
+export function isCardBearingType(type?: string): boolean {
+  return type ? CARD_BEARING_TYPES.has(type) : false
+}
+```
+
+```vue
+<!-- MessageShell.vue -->
+const isCardBearing = computed(() => isCardBearingType(props.messageType))
+```
+
+```css
+/* F4-A: card-bearing types skip bubble chrome */
+.message-shell--assistant.message-shell--card .message-shell__content {
+  background: transparent;
+  border: none;
+  border-radius: 0;
+  padding: 0;
+}
+```
+
+**Important**: `error` type must be in the exclusion set — `ErrorCard.vue` ships
+full chrome (background + border + border-radius + padding). Without exclusion,
+the assistant bubble wraps ErrorCard producing double-background regression.
+
+### 3. Extract computed logic to pure functions for testability
+
+When project convention avoids `@vue/test-utils`, extract computed logic to
+pure functions in a `helpers/` module. The Vue computed becomes a thin wrapper
+preserving reactivity:
+
+```ts
+// bubbleUtils.ts
+export function isAssistantBubbleEmpty(message: IChatMessage): boolean {
+  if (message.role !== 'assistant') return false
+  return (
+    !message.content &&
+    !message.thinking &&
+    (!message.tool_calls || message.tool_calls.length === 0)
+  )
+}
+
+// ChatMessage.vue — thin reactive wrapper
+import { isAssistantBubbleEmpty } from './helpers/bubbleUtils'
+const isBubbleEmpty = computed(() => isAssistantBubbleEmpty(props.message))
+```
+
+This pattern lets you test the logic with plain vitest:
+
+```ts
+// bubbleUtils.test.ts
+it('returns true for assistant with no content, no thinking, no tool_calls', () => {
+  const msg = makeMsg({ role: 'assistant', content: '' })
+  expect(isAssistantBubbleEmpty(msg)).toBe(true)
+})
+```
+
+## Why This Matters
+
+- **`:empty` failure mode is silent**: CSS doesn't error, the selector just
+  never matches. Empty bubbles render with full chrome — visually broken but
+  not detectable without manual inspection. A P0 review finding.
+- **Double-chrome regression**: ErrorCard wrapping was missed in initial F4-A
+  planning (only 9 types listed). The `error` type was added during code review
+  (P2 finding) after spotting ErrorCard's full chrome.
+- **Test coverage of decisions**: G1 (`:empty` replacement) and F4-A
+  (card exclusion) are P0 decisions in the plan. Without extracted pure
+  functions, these decisions have no unit test coverage — only visual
+  verification. Extracting to `bubbleUtils.ts` added 36 tests covering all
+  edge cases.
+
+## When to Apply
+
+- **Any Vue component where `:empty` seems tempting**: Don't use it. Vue
+  components render root elements. Use JS computed.
+- **Card-vs-bubble styling decisions**: When a component renders different
+  "chrome" levels (full card vs. bubble vs. plain), use a `messageType` prop
+  + Set lookup. Avoid `:has()` — it couples styling to DOM structure.
+- **Computed properties with non-trivial logic**: If the logic is a P0/P1
+  decision (G1, F4-A) or has edge cases (undefined fields, empty arrays),
+  extract to a pure function. The Vue computed should be a one-line wrapper.
+- **Projects without `@vue/test-utils`**: Pure-function extraction is the
+  primary path to computed-property test coverage.
+
+## Examples
+
+### Before (broken — `:empty` never matches)
+
+```vue
+<!-- AssistantText always renders a root div, so :empty never matches -->
+<style scoped>
+.message-shell--assistant .message-shell__content:empty {
+  display: none;
+}
+</style>
+```
+
+### After (working — JS computed via pure function)
+
+```vue
+<template>
+  <MessageShell :is-empty="isBubbleEmpty" :message-type="spec.type">
+    <AssistantText :content="message.content" />
+  </MessageShell>
+</template>
+
+<script setup lang="ts">
+import { isAssistantBubbleEmpty } from './helpers/bubbleUtils'
+const isBubbleEmpty = computed(() => isAssistantBubbleEmpty(props.message))
+</script>
+```
+
+### Card-bearing exclusion — complete type list
+
+```ts
+// All 10 types that ship their own chrome and must skip the bubble:
+const CARD_BEARING_TYPES = new Set<string>([
+  'board_conclusion',      // BoardConclusionCard — full chrome
+  'team_plan',             // TeamPlanCard — full chrome
+  'debate_started',        // DebateBannerCard — full chrome (plan typo'd as debate_banner)
+  'debate_argument',       // DebateArgumentCard — partial chrome
+  'debate_summary',        // DebateSummaryCard — partial chrome
+  'debate_resolved',       // DebateResolvedCard — partial chrome
+  'collaboration_graph',   // CollaborationGraph — partial chrome
+  'review_result',         // ReviewResultCard — partial chrome
+  'risk_flagged',          // RiskFlaggedCard — partial chrome
+  'error',                 // ErrorCard — full chrome (added in code review P2)
+])
+```
+
+### Test file structure (pure-TS, no @vue/test-utils)
+
+```ts
+// tests/unit/helpers/bubbleUtils.test.ts
+import { describe, expect, it } from 'vitest'
+import { isCardBearingType, isAssistantBubbleEmpty } from '@/components/chat/helpers/bubbleUtils'
+import type { IChatMessage } from '@/api/types'
+
+function makeMsg(overrides: Partial<IChatMessage> = {}): IChatMessage {
+  return { id: 'm1', role: 'assistant', content: '', timestamp: '...', ...overrides }
+}
+
+describe('isCardBearingType', () => {
+  it.each(['board_conclusion', 'error', 'team_plan'])('returns true for %s', (type) => {
+    expect(isCardBearingType(type)).toBe(true)
+  })
+  it('returns false for undefined', () => {
+    expect(isCardBearingType(undefined)).toBe(false)
+  })
+})
+```