fix: anchor board zoom to pointer

src/boardDomain.test.ts

@@ -16,6 +16,7 @@ import {
   timelineRange,
   zoomFromWheel,
   zoomFromPinch,
+  zoomViewportAt,
 } from './boardDomain'
 
 describe('red thread geometry', () => {
@@ -80,6 +81,15 @@ describe('board coordinate math', () => {
     expect(zoomFromPinch(1, 0, 120)).toBe(1)
   })
 
+  it('keeps the board point beneath the cursor fixed while zooming', () => {
+    const anchor = { x: 110, y: 120 }
+    const zoomed = zoomViewportAt(state.viewport, 1, anchor)
+    expect(zoomed).toEqual({ x: -90, y: -80, zoom: 1 })
+    expect(zoomed.x + 200 * zoomed.zoom).toBe(anchor.x)
+    expect(zoomed.y + 200 * zoomed.zoom).toBe(anchor.y)
+    expect(zoomViewportAt(zoomed, .5, anchor)).toEqual(state.viewport)
+  })
+
   it('places new exhibits in deterministic open slots', () => {
     const preferred = { x: 500, y: 400 }
     expect(nextOpenBoardPosition([], preferred, { width: 280 })).toEqual(preferred)