fix: bypass CodeMirror posAtCoords for accurate cursor at non-100% CSS zoom

CSS zoom on document.documentElement causes a coordinate space mismatch
between mouse event clientX/Y (viewport space) and Range.getClientRects()
(CSS space), breaking CodeMirror's click-to-position mapping. The previous
requestMeasure() fix only recalibrated cached geometry, not this mismatch.

New approach: zoomCursorFix extension patches posAtCoords/posAndSideAtCoords
on the EditorView instance to use document.caretRangeFromPoint() — the
browser's native, zoom-aware API — with coord-adjustment fallback.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/extensions/zoomCursorFix.test.ts

@@ -0,0 +1,88 @@
+import { describe, it, expect, vi, afterEach } from 'vitest'
+import { getDocumentZoom, adjustCoordsForZoom } from './zoomCursorFix'
+
+function mockComputedZoom(value: string) {
+  const real = window.getComputedStyle.bind(window)
+  return vi.spyOn(window, 'getComputedStyle').mockImplementation((elt, pseudo) => {
+    const style = real(elt, pseudo)
+    if (elt === document.documentElement) {
+      return new Proxy(style, {
+        get(target, prop) {
+          if (prop === 'zoom') return value
+          const val = Reflect.get(target, prop)
+          return typeof val === 'function' ? val.bind(target) : val
+        },
+      })
+    }
+    return style
+  })
+}
+
+describe('getDocumentZoom', () => {
+  afterEach(() => {
+    vi.restoreAllMocks()
+  })
+
+  it('returns 1 when no zoom is set', () => {
+    expect(getDocumentZoom()).toBe(1)
+  })
+
+  it('returns the zoom factor when computed style reports a decimal', () => {
+    const spy = mockComputedZoom('1.5')
+    expect(getDocumentZoom()).toBe(1.5)
+    spy.mockRestore()
+  })
+
+  it('returns the zoom factor for sub-100% zoom', () => {
+    const spy = mockComputedZoom('0.8')
+    expect(getDocumentZoom()).toBe(0.8)
+    spy.mockRestore()
+  })
+
+  it('returns 1 for zoom: normal', () => {
+    const spy = mockComputedZoom('normal')
+    expect(getDocumentZoom()).toBe(1)
+    spy.mockRestore()
+  })
+
+  it('returns 1 for empty/missing zoom value', () => {
+    const spy = mockComputedZoom('')
+    expect(getDocumentZoom()).toBe(1)
+    spy.mockRestore()
+  })
+})
+
+describe('adjustCoordsForZoom', () => {
+  it('returns coords unchanged when zoom is 1', () => {
+    expect(adjustCoordsForZoom({ x: 200, y: 100 }, 1)).toEqual({ x: 200, y: 100 })
+  })
+
+  it('divides coords by zoom factor for zoom > 1', () => {
+    const result = adjustCoordsForZoom({ x: 300, y: 150 }, 1.5)
+    expect(result.x).toBe(200)
+    expect(result.y).toBe(100)
+  })
+
+  it('divides coords by zoom factor for zoom < 1', () => {
+    const result = adjustCoordsForZoom({ x: 160, y: 80 }, 0.8)
+    expect(result.x).toBe(200)
+    expect(result.y).toBe(100)
+  })
+
+  it('handles common zoom levels correctly', () => {
+    // 90% zoom
+    const at90 = adjustCoordsForZoom({ x: 90, y: 90 }, 0.9)
+    expect(at90.x).toBeCloseTo(100, 10)
+    expect(at90.y).toBeCloseTo(100, 10)
+
+    // 110% zoom
+    const at110 = adjustCoordsForZoom({ x: 110, y: 110 }, 1.1)
+    expect(at110.x).toBeCloseTo(100, 10)
+    expect(at110.y).toBeCloseTo(100, 10)
+
+    // 125% zoom
+    const at125 = adjustCoordsForZoom({ x: 125, y: 125 }, 1.25)
+    expect(at125.x).toBeCloseTo(100, 10)
+    expect(at125.y).toBeCloseTo(100, 10)
+  })
+})