fix: anchor board zoom to pointer

src/App.tsx

@@ -1,7 +1,7 @@
 import { useCallback, useEffect, useLayoutEffect, useMemo, useRef, useState } from 'react'
 import { BookOpen, Building2, CalendarClock, ChevronRight, CircleHelp, FileText, FolderOpen, Hand, Image as ImageIcon, Link2, Minus, MousePointer2, Network, NotebookPen, Pencil, Plus, RotateCcw, Search, Trash2, Upload, UserRound, X, ZoomIn, ZoomOut } from 'lucide-react'
 import type { BriefConcept, CaseDocument, CaseState, Connection, Evidence, LevelBrief, OrganizationKind, PartyKind, SourceFileType, TimelineRange, WidgetRelation } from './types'
-import { clampBoardZoom, containedIds, dateValue, folderIsOpen, moveBoardPoint, nextOpenBoardPosition, normalizeCase, panViewport, relationPosition, threadCurve, timelinePositionPercent, timelineRange, zoomFromPinch, zoomFromWheel } from './boardDomain'
+import { clampBoardZoom, containedIds, dateValue, folderIsOpen, moveBoardPoint, nextOpenBoardPosition, normalizeCase, panViewport, relationPosition, threadCurve, timelinePositionPercent, timelineRange, zoomFromPinch, zoomFromWheel, zoomViewportAt } from './boardDomain'
 import { documentWidget, exhibitWidget } from './exhibitRegistry'
 
 const BOARD_W = 2400
@@ -440,7 +440,9 @@ function Board({ state, selected, linkFrom, recentlyCreatedExhibitId, recentlyCr
       event.preventDefault()
       event.stopPropagation()
       if (event.ctrlKey || event.metaKey || event.deltaY === 0) return
-      update(s => ({ ...s, viewport: { ...s.viewport, zoom: zoomFromWheel(s.viewport.zoom, event.deltaY) } }))
+      const bounds = board.getBoundingClientRect()
+      const anchor = { x: event.clientX - bounds.left, y: event.clientY - bounds.top }
+      update(s => ({ ...s, viewport: zoomViewportAt(s.viewport, zoomFromWheel(s.viewport.zoom, event.deltaY), anchor) }))
     }
     board.addEventListener('wheel', handleWheelZoom, { passive: false })
     return () => board.removeEventListener('wheel', handleWheelZoom)
@@ -477,7 +479,11 @@ function Board({ state, selected, linkFrom, recentlyCreatedExhibitId, recentlyCr
         const points = [...touchPoints.current.values()]
         const distance = Math.hypot(points[1].x - points[0].x, points[1].y - points[0].y)
         const previous = pinchDistance.current
-        if (previous) update(s => ({ ...s, viewport: { ...s.viewport, zoom: zoomFromPinch(s.viewport.zoom, previous, distance) } }))
+        if (previous && boardRef.current) {
+          const bounds = boardRef.current.getBoundingClientRect()
+          const anchor = { x: (points[0].x + points[1].x) / 2 - bounds.left, y: (points[0].y + points[1].y) / 2 - bounds.top }
+          update(s => ({ ...s, viewport: zoomViewportAt(s.viewport, zoomFromPinch(s.viewport.zoom, previous, distance), anchor) }))
+        }
         pinchDistance.current = distance
         suppressClick.current = true
         event.preventDefault()

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)

src/boardDomain.ts

@@ -31,6 +31,13 @@ export function zoomFromPinch(currentZoom: number, previousDistance: number, nex
   return clampBoardZoom(currentZoom * nextDistance / previousDistance)
 }
 
+export function zoomViewportAt(viewport: Viewport, nextZoom: number, screenAnchor: BoardPoint): Viewport {
+  if (!Number.isFinite(viewport.zoom) || viewport.zoom <= 0) throw new RangeError('Board zoom must be positive')
+  const zoom = clampBoardZoom(nextZoom)
+  const boardPoint = { x: (screenAnchor.x - viewport.x) / viewport.zoom, y: (screenAnchor.y - viewport.y) / viewport.zoom }
+  return { x: screenAnchor.x - boardPoint.x * zoom, y: screenAnchor.y - boardPoint.y * zoom, zoom }
+}
+
 export function moveBoardPoint(origin: { x: number; y: number }, screenDelta: { x: number; y: number }, zoom: number) {
   if (!Number.isFinite(zoom) || zoom <= 0) throw new RangeError('Board zoom must be positive')
   return { x: origin.x + screenDelta.x / zoom, y: origin.y + screenDelta.y / zoom }