test: add unit tests for ChatHeightCache and chatAnchoring

31 vitest unit tests covering:
- ChatHeightCache: get/set/delete/has/clear/size/version tracking
- calculateTotalHeight: empty, estimated, measured, mixed
- calculateOffsetForIndex: boundaries, mixed heights, clamping
- captureScrollAnchor: empty, top, partway, estimated, fallback
- restoreScrollAnchor: missing anchor, exact restore, prepend,
  negative offset

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jaysin586

src/lib/virtual-chat/chatAnchoring.test.ts

@@ -0,0 +1,133 @@
+import { describe, expect, it } from 'vitest'
+import { captureScrollAnchor, restoreScrollAnchor } from './chatAnchoring.js'
+import { ChatHeightCache } from './chatMeasurement.svelte.js'
+
+type Msg = { id: string }
+const getId = (m: Msg) => m.id
+const msgs = (ids: string[]): Msg[] => ids.map((id) => ({ id }))
+
+describe('captureScrollAnchor', () => {
+    it('returns null for empty messages', () => {
+        const cache = new ChatHeightCache()
+        expect(captureScrollAnchor([], getId, cache, 40, 0)).toBeNull()
+    })
+
+    it('captures the first visible message at scrollTop=0', () => {
+        const cache = new ChatHeightCache()
+        cache.set('1', 50)
+        cache.set('2', 50)
+        cache.set('3', 50)
+        const m = msgs(['1', '2', '3'])
+
+        const anchor = captureScrollAnchor(m, getId, cache, 50, 0)
+        expect(anchor).not.toBeNull()
+        expect(anchor!.messageId).toBe('1')
+        expect(anchor!.offsetFromViewportTop).toBe(0)
+    })
+
+    it('captures correct message when scrolled partway', () => {
+        const cache = new ChatHeightCache()
+        cache.set('1', 50)
+        cache.set('2', 50)
+        cache.set('3', 50)
+        const m = msgs(['1', '2', '3'])
+
+        // scrollTop=60 means message 1 (0-50) is above viewport,
+        // message 2 (50-100) is the first visible
+        const anchor = captureScrollAnchor(m, getId, cache, 50, 60)
+        expect(anchor!.messageId).toBe('2')
+        // message 2 starts at offset 50, scrollTop=60, so offset from viewport top = 50 - 60 = -10
+        expect(anchor!.offsetFromViewportTop).toBe(-10)
+    })
+
+    it('uses estimated height for unmeasured messages', () => {
+        const cache = new ChatHeightCache()
+        const m = msgs(['1', '2', '3'])
+
+        // All unmeasured at estimated=40. scrollTop=50 means:
+        // msg 1 (0-40) fully above, msg 2 (40-80) is first visible
+        const anchor = captureScrollAnchor(m, getId, cache, 40, 50)
+        expect(anchor!.messageId).toBe('2')
+        expect(anchor!.offsetFromViewportTop).toBe(40 - 50)
+    })
+
+    it('falls back to last message if scrolled past all', () => {
+        const cache = new ChatHeightCache()
+        cache.set('1', 50)
+        cache.set('2', 50)
+        const m = msgs(['1', '2'])
+
+        // scrollTop=200 is past all content (total = 100)
+        const anchor = captureScrollAnchor(m, getId, cache, 50, 200)
+        expect(anchor!.messageId).toBe('2')
+    })
+})
+
+describe('restoreScrollAnchor', () => {
+    it('returns 0 if anchor message is not found', () => {
+        const cache = new ChatHeightCache()
+        const anchor = { messageId: 'nonexistent', offsetFromViewportTop: 0 }
+        const m = msgs(['1', '2'])
+
+        expect(restoreScrollAnchor(anchor, m, getId, cache, 40)).toBe(0)
+    })
+
+    it('restores exact scrollTop for anchor at top of viewport', () => {
+        const cache = new ChatHeightCache()
+        cache.set('1', 50)
+        cache.set('2', 50)
+        cache.set('3', 50)
+        const m = msgs(['1', '2', '3'])
+
+        // Anchor: message 2 was at offset 0 from viewport top
+        const anchor = { messageId: '2', offsetFromViewportTop: 0 }
+        // Message 2 is at offset 50 (after message 1)
+        // scrollTop = 50 - 0 = 50
+        expect(restoreScrollAnchor(anchor, m, getId, cache, 50)).toBe(50)
+    })
+
+    it('accounts for offsetFromViewportTop', () => {
+        const cache = new ChatHeightCache()
+        cache.set('1', 50)
+        cache.set('2', 50)
+        cache.set('3', 50)
+        const m = msgs(['1', '2', '3'])
+
+        // Anchor: message 2 was 10px below the viewport top
+        const anchor = { messageId: '2', offsetFromViewportTop: 10 }
+        // Message 2 is at offset 50. scrollTop = 50 - 10 = 40
+        expect(restoreScrollAnchor(anchor, m, getId, cache, 50)).toBe(40)
+    })
+
+    it('restores correctly after prepending messages', () => {
+        const cache = new ChatHeightCache()
+        // Original: messages 3, 4, 5 with measured heights
+        cache.set('3', 60)
+        cache.set('4', 60)
+        cache.set('5', 60)
+
+        // Before prepend: anchor was message 3 at viewport top
+        const anchor = { messageId: '3', offsetFromViewportTop: 0 }
+
+        // After prepend: messages 1, 2, 3, 4, 5
+        cache.set('1', 50)
+        cache.set('2', 50)
+        const afterPrepend = msgs(['1', '2', '3', '4', '5'])
+
+        // Message 3 is now at offset 50 + 50 = 100
+        // scrollTop = 100 - 0 = 100
+        expect(restoreScrollAnchor(anchor, afterPrepend, getId, cache, 40)).toBe(100)
+    })
+
+    it('handles negative offsetFromViewportTop', () => {
+        const cache = new ChatHeightCache()
+        cache.set('1', 50)
+        cache.set('2', 50)
+        const m = msgs(['1', '2'])
+
+        // Message 1 was partially scrolled out: its top was 10px above viewport
+        const anchor = { messageId: '1', offsetFromViewportTop: -10 }
+        // Message 1 is at offset 0. scrollTop = 0 - (-10) = 10
+        expect(restoreScrollAnchor(anchor, m, getId, cache, 50)).toBe(10)
+    })
+})

src/lib/virtual-chat/chatMeasurement.svelte.test.ts

@@ -0,0 +1,175 @@
+import { describe, expect, it } from 'vitest'
+import {
+    ChatHeightCache,
+    calculateOffsetForIndex,
+    calculateTotalHeight
+} from './chatMeasurement.svelte.js'
+
+type Msg = { id: string }
+const getId = (m: Msg) => m.id
+const msgs = (ids: string[]): Msg[] => ids.map((id) => ({ id }))
+
+describe('ChatHeightCache', () => {
+    it('returns undefined for unmeasured messages', () => {
+        const cache = new ChatHeightCache()
+        expect(cache.get('a')).toBeUndefined()
+    })
+
+    it('stores and retrieves heights', () => {
+        const cache = new ChatHeightCache()
+        cache.set('a', 100)
+        expect(cache.get('a')).toBe(100)
+    })
+
+    it('returns true when height changes', () => {
+        const cache = new ChatHeightCache()
+        expect(cache.set('a', 100)).toBe(true)
+        expect(cache.set('a', 200)).toBe(true)
+    })
+
+    it('returns false when height is the same', () => {
+        const cache = new ChatHeightCache()
+        cache.set('a', 100)
+        expect(cache.set('a', 100)).toBe(false)
+    })
+
+    it('tracks size correctly', () => {
+        const cache = new ChatHeightCache()
+        expect(cache.size).toBe(0)
+        cache.set('a', 50)
+        expect(cache.size).toBe(1)
+        cache.set('b', 60)
+        expect(cache.size).toBe(2)
+    })
+
+    it('has() works correctly', () => {
+        const cache = new ChatHeightCache()
+        expect(cache.has('a')).toBe(false)
+        cache.set('a', 50)
+        expect(cache.has('a')).toBe(true)
+    })
+
+    it('delete removes an entry', () => {
+        const cache = new ChatHeightCache()
+        cache.set('a', 50)
+        cache.set('b', 60)
+        cache.delete('a')
+        expect(cache.has('a')).toBe(false)
+        expect(cache.get('a')).toBeUndefined()
+        expect(cache.size).toBe(1)
+    })
+
+    it('delete on nonexistent key is a no-op', () => {
+        const cache = new ChatHeightCache()
+        const vBefore = cache.version
+        cache.delete('nonexistent')
+        expect(cache.version).toBe(vBefore)
+    })
+
+    it('clear removes all entries', () => {
+        const cache = new ChatHeightCache()
+        cache.set('a', 50)
+        cache.set('b', 60)
+        cache.clear()
+        expect(cache.size).toBe(0)
+        expect(cache.has('a')).toBe(false)
+        expect(cache.has('b')).toBe(false)
+    })
+
+    it('version increments on set, delete, clear', () => {
+        const cache = new ChatHeightCache()
+        const v0 = cache.version
+        cache.set('a', 50)
+        expect(cache.version).toBe(v0 + 1)
+        cache.set('a', 60)
+        expect(cache.version).toBe(v0 + 2)
+        cache.delete('a')
+        expect(cache.version).toBe(v0 + 3)
+        cache.set('b', 70)
+        cache.clear()
+        expect(cache.version).toBe(v0 + 5)
+    })
+
+    it('version does not increment on no-op set', () => {
+        const cache = new ChatHeightCache()
+        cache.set('a', 50)
+        const v = cache.version
+        cache.set('a', 50)
+        expect(cache.version).toBe(v)
+    })
+})
+
+describe('calculateTotalHeight', () => {
+    it('returns 0 for empty messages', () => {
+        const cache = new ChatHeightCache()
+        expect(calculateTotalHeight([], getId, cache, 40)).toBe(0)
+    })
+
+    it('uses estimated height for unmeasured messages', () => {
+        const cache = new ChatHeightCache()
+        const m = msgs(['1', '2', '3'])
+        expect(calculateTotalHeight(m, getId, cache, 40)).toBe(120)
+    })
+
+    it('uses measured height when available', () => {
+        const cache = new ChatHeightCache()
+        cache.set('1', 100)
+        cache.set('2', 50)
+        const m = msgs(['1', '2', '3'])
+        // 100 + 50 + 40(estimated)
+        expect(calculateTotalHeight(m, getId, cache, 40)).toBe(190)
+    })
+
+    it('uses all measured heights when fully measured', () => {
+        const cache = new ChatHeightCache()
+        cache.set('1', 80)
+        cache.set('2', 60)
+        cache.set('3', 100)
+        const m = msgs(['1', '2', '3'])
+        expect(calculateTotalHeight(m, getId, cache, 40)).toBe(240)
+    })
+})
+
+describe('calculateOffsetForIndex', () => {
+    it('returns 0 for index 0', () => {
+        const cache = new ChatHeightCache()
+        const m = msgs(['1', '2', '3'])
+        expect(calculateOffsetForIndex(m, 0, getId, cache, 40)).toBe(0)
+    })
+
+    it('sums estimated heights for unmeasured messages', () => {
+        const cache = new ChatHeightCache()
+        const m = msgs(['1', '2', '3', '4'])
+        // offset for index 2 = height[0] + height[1] = 40 + 40
+        expect(calculateOffsetForIndex(m, 2, getId, cache, 40)).toBe(80)
+    })
+
+    it('uses measured heights when available', () => {
+        const cache = new ChatHeightCache()
+        cache.set('1', 100)
+        cache.set('2', 50)
+        const m = msgs(['1', '2', '3'])
+        // offset for index 2 = 100 + 50
+        expect(calculateOffsetForIndex(m, 2, getId, cache, 40)).toBe(150)
+    })
+
+    it('mixes measured and estimated', () => {
+        const cache = new ChatHeightCache()
+        cache.set('2', 80)
+        const m = msgs(['1', '2', '3', '4'])
+        // offset for index 3 = 40(est) + 80(measured) + 40(est)
+        expect(calculateOffsetForIndex(m, 3, getId, cache, 40)).toBe(160)
+    })
+
+    it('clamps to messages length', () => {
+        const cache = new ChatHeightCache()
+        const m = msgs(['1', '2'])
+        // index 10 is beyond array — should sum all heights
+        expect(calculateOffsetForIndex(m, 10, getId, cache, 40)).toBe(80)
+    })
+
+    it('returns 0 for empty messages', () => {
+        const cache = new ChatHeightCache()
+        expect(calculateOffsetForIndex([], 5, getId, cache, 40)).toBe(0)
+    })
+})