fix: clear stale compare sync offsets on version switch

apps/web/components/review/compare/__tests__/compare-overlay.test.tsx

@@ -1,5 +1,5 @@
 import { describe, expect, it, vi, beforeEach } from 'vitest'
-import { fireEvent, render, screen, waitFor } from '@testing-library/react'
+import { fireEvent, render, screen, waitFor, within } from '@testing-library/react'
 import { useReviewStore } from '@/stores/review-store'
 
 const replace = vi.fn()
@@ -315,6 +315,48 @@ describe('CompareOverlay marker click', () => {
   })
 })
 
+describe('CompareOverlay version-switch offset reset (#182)', () => {
+  it('switching the left version clears offA/offB (calibrated for the OLD pair)', () => {
+    // Pre-existing sync offsets from calibrating the v1/v3 pair.
+    searchParamsString = 'compare=v-1&offA=1.5&offB=-0.5'
+    render(
+      <CompareOverlay
+        asset={videoAsset}
+        versions={[makeVersion(1), makeVersion(2), makeVersion(3)]}
+        rightVersion={makeVersion(3)}
+        onClose={vi.fn()}
+      />,
+    )
+
+    fireEvent.click(within(screen.getByTestId('compare-select-a')).getByRole('button'))
+    fireEvent.click(screen.getByRole('option', { name: /^v2$/ }))
+
+    const url = new URL(replace.mock.calls.at(-1)?.[0], 'http://x')
+    expect(url.searchParams.get('offA')).toBeNull()
+    expect(url.searchParams.get('offB')).toBeNull()
+    expect(url.searchParams.get('compare')).toBe('v-2')
+  })
+
+  it('switching the right version clears offA/offB too', () => {
+    searchParamsString = 'compare=v-1&offA=1.5&offB=-0.5'
+    render(
+      <CompareOverlay
+        asset={videoAsset}
+        versions={[makeVersion(1), makeVersion(2), makeVersion(3)]}
+        rightVersion={makeVersion(3)}
+        onClose={vi.fn()}
+      />,
+    )
+
+    fireEvent.click(within(screen.getByTestId('compare-select-b')).getByRole('button'))
+    fireEvent.click(screen.getByRole('option', { name: /^v2$/ }))
+
+    const url = new URL(replace.mock.calls.at(-1)?.[0], 'http://x')
+    expect(url.searchParams.get('offA')).toBeNull()
+    expect(url.searchParams.get('offB')).toBeNull()
+  })
+})
+
 describe('CompareOverlay per-pane annotation display', () => {
   const DRAWING = { objects: [], _canvasWidth: 640, _canvasHeight: 360 }
 

apps/web/components/review/compare/compare-overlay.tsx

@@ -279,6 +279,23 @@ export function CompareOverlay({ asset, versions, rightVersion, onClose, canComm
     [sideB],
   )
 
+  // Version switch (either pane): offA/offB were calibrated for the OLD pair —
+  // they no longer describe the new one, so drop them rather than silently
+  // misapplying a stale sync offset to the new pair (#182).
+  const handleSwitchLeft = React.useCallback(
+    (v: AssetVersion) => {
+      writeParams((p) => { p.set('compare', v.id); p.delete('offA'); p.delete('offB') })
+    },
+    [writeParams],
+  )
+  const handleSwitchRight = React.useCallback(
+    (v: AssetVersion) => {
+      writeParams((p) => { p.delete('offA'); p.delete('offB') })
+      setCurrentVersion(v)
+    },
+    [writeParams, setCurrentVersion],
+  )
+
   // Shared zoom/pan for image modes
   const transform = useSharedTransform()
 
@@ -310,7 +327,7 @@ export function CompareOverlay({ asset, versions, rightVersion, onClose, canComm
             value={left.id}
             excludeId={right.id}
             accentClass="text-sky-400"
-            onChange={(v) => writeParams((p) => p.set('compare', v.id))}
+            onChange={handleSwitchLeft}
           />
         </div>
         {/* Center: asset title (flex-1 min-w-0 truncates without pushing either
@@ -338,7 +355,7 @@ export function CompareOverlay({ asset, versions, rightVersion, onClose, canComm
             value={right.id}
             excludeId={left.id}
             accentClass="text-emerald-400"
-            onChange={(v) => setCurrentVersion(v)}
+            onChange={handleSwitchRight}
           />
           <button
             type="button"