fix: cache canvas cursor style to avoid redundant DOM writes (#9171)

## Summary

Cache `canvas.style.cursor` to avoid redundant DOM writes that dirty
Firefox's style tree.

## Changes

- **What**: Add `_lastCursor` field to
`LGraphCanvas._updateCursorStyle()` — only writes `canvas.style.cursor`
when the value changes. Eliminates ~347 redundant style mutations per
profiling session.

## Review Focus

- The fix is 2 lines (cache field + comparison). The unit test validates
the caching pattern without requiring full LGraphCanvas instantiation.
- This is one of several contributors to Firefox's cascading style
recalculation freeze. Each `canvas.style.cursor` write dirties the style
tree, which is flushed during the next paint in the canvas render loop.

## Stack

2 of 4 in Firefox perf fix stack. Depends on #9170.

<!-- Fixes #ISSUE_NUMBER -->

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9171-fix-cache-canvas-cursor-style-to-avoid-redundant-DOM-writes-3116d73d36508139827fe1d644fa1bd0)
by [Unito](https://www.unito.io)

src/lib/litegraph/src/LGraphCanvas.ts

@@ -12,6 +12,7 @@ import { forEachNode } from '@/utils/graphTraversalUtil'
 
 import { CanvasPointer } from './CanvasPointer'
 import type { ContextMenu } from './ContextMenu'
+import { createCursorCache } from './cursorCache'
 import { DragAndScale } from './DragAndScale'
 import type { AnimationOptions } from './DragAndScale'
 import type { LGraph } from './LGraph'
@@ -364,6 +365,8 @@ export class LGraphCanvas implements CustomEventDispatcher<LGraphCanvasEventMap>
     this.canvas.dispatchEvent(new CustomEvent(type, { detail }))
   }
 
+  private _setCursor!: ReturnType<typeof createCursorCache>
+
   private _updateCursorStyle() {
     if (!this.state.shouldSetCursor) return
 
@@ -386,7 +389,7 @@ export class LGraphCanvas implements CustomEventDispatcher<LGraphCanvasEventMap>
       cursor = 'grab'
     }
 
-    this.canvas.style.cursor = cursor
+    this._setCursor(cursor)
   }
 
   // Whether the canvas was previously being dragged prior to pressing space key.
@@ -1911,6 +1914,7 @@ export class LGraphCanvas implements CustomEventDispatcher<LGraphCanvasEventMap>
     this.pointer.element = element
 
     if (!element) return
+    this._setCursor = createCursorCache(element)
 
     // TODO: classList.add
     element.className += ' lgraphcanvas'
@@ -2970,7 +2974,7 @@ export class LGraphCanvas implements CustomEventDispatcher<LGraphCanvasEventMap>
           }
 
           // Set appropriate cursor for resize direction
-          this.canvas.style.cursor = cursors[resizeDirection]
+          this._setCursor(cursors[resizeDirection])
           return
         }
       }

src/lib/litegraph/src/cursorCache.test.ts

@@ -0,0 +1,59 @@
+import { describe, expect, it, vi } from 'vitest'
+
+import { createCursorCache } from './cursorCache'
+
+function createMockElement() {
+  let cursorValue = ''
+  const setter = vi.fn((value: string) => {
+    cursorValue = value
+  })
+
+  const element = document.createElement('div')
+  Object.defineProperty(element.style, 'cursor', {
+    get: () => cursorValue,
+    set: setter
+  })
+
+  return { element, setter }
+}
+
+describe('createCursorCache', () => {
+  it('should only write to DOM when cursor value changes', () => {
+    const { element, setter } = createMockElement()
+    const setCursor = createCursorCache(element)
+
+    setCursor('crosshair')
+    setCursor('crosshair')
+    setCursor('crosshair')
+
+    expect(setter).toHaveBeenCalledTimes(1)
+    expect(setter).toHaveBeenCalledWith('crosshair')
+  })
+
+  it('should write to DOM when cursor value differs', () => {
+    const { element, setter } = createMockElement()
+    const setCursor = createCursorCache(element)
+
+    setCursor('default')
+    setCursor('crosshair')
+    setCursor('grabbing')
+
+    expect(setter).toHaveBeenCalledTimes(3)
+    expect(setter).toHaveBeenNthCalledWith(1, 'default')
+    expect(setter).toHaveBeenNthCalledWith(2, 'crosshair')
+    expect(setter).toHaveBeenNthCalledWith(3, 'grabbing')
+  })
+
+  it('should skip repeated values interspersed with changes', () => {
+    const { element, setter } = createMockElement()
+    const setCursor = createCursorCache(element)
+
+    setCursor('default')
+    setCursor('default')
+    setCursor('grab')
+    setCursor('grab')
+    setCursor('default')
+
+    expect(setter).toHaveBeenCalledTimes(3)
+  })
+})

src/lib/litegraph/src/cursorCache.ts

@@ -0,0 +1,8 @@
+export function createCursorCache(element: HTMLElement) {
+  let lastCursor = ''
+  return function setCursor(cursor: string) {
+    if (cursor === lastCursor) return
+    lastCursor = cursor
+    element.style.cursor = cursor
+  }
+}