diff --git a/packages/page-controller/src/PageController.ts b/packages/page-controller/src/PageController.ts
index d168e4d..c30f508 100644
--- a/packages/page-controller/src/PageController.ts
+++ b/packages/page-controller/src/PageController.ts
@@ -14,7 +14,6 @@ import {
 	scrollVertically,
 	selectOptionElement,
 } from './actions'
-import { VIEWPORT_EXPANSION } from './constants'
 import * as dom from './dom'
 import type { FlatDomTree, InteractiveElementDomNode } from './dom/dom_tree/type'
 import { getPageInfo } from './dom/getPageInfo'
@@ -24,7 +23,6 @@ import { patchReact } from './patches/react'
  * Configuration for PageController
  */
 export interface PageControllerConfig extends dom.DomConfig {
-	viewportExpansion?: number
 	/** Enable visual mask overlay during operations (default: false) */
 	enableMask?: boolean
 }
@@ -131,7 +129,7 @@ export class PageController extends EventTarget {
 		const url = window.location.href
 		const title = document.title
 		const pi = getPageInfo()
-		const viewportExpansion = this.config.viewportExpansion ?? VIEWPORT_EXPANSION
+		const viewportExpansion = dom.resolveViewportExpansion(this.config.viewportExpansion)
 
 		await this.updateTree()
 
diff --git a/packages/page-controller/src/constants.ts b/packages/page-controller/src/constants.ts
deleted file mode 100644
index ebe131e..0000000
--- a/packages/page-controller/src/constants.ts
+++ /dev/null
@@ -1,16 +0,0 @@
-/**
- * Copyright (C) 2025 Alibaba Group Holding Limited
- * All rights reserved.
- */
-
-/**
- * Viewport expansion for DOM tree extraction.
- * -1 means full page (no viewport restriction)
- * 0 means viewport only
- * positive values expand the viewport by that many pixels
- *
- * @note Since isTopElement depends on elementFromPoint,
- * it returns null when out of viewport, this feature has no practical use, only differ between -1 and 0
- */
-// export const VIEWPORT_EXPANSION = 100
-export const VIEWPORT_EXPANSION = -1
diff --git a/packages/page-controller/src/dom/index.ts b/packages/page-controller/src/dom/index.ts
index f8e2608..2968c0c 100644
--- a/packages/page-controller/src/dom/index.ts
+++ b/packages/page-controller/src/dom/index.ts
@@ -1,4 +1,3 @@
-import { VIEWPORT_EXPANSION } from '../constants'
 import domTree from './dom_tree/index.js'
 import {
 	ElementDomNode,
@@ -7,7 +6,23 @@ import {
 	TextDomNode,
 } from './dom_tree/type'
 
+/**
+ * Viewport expansion for DOM tree extraction.
+ * -1 means full page (no viewport restriction)
+ * 0 means viewport only
+ * positive values expand the viewport by that many pixels
+ *
+ * @note Since isTopElement depends on elementFromPoint,
+ * it returns null when out of viewport, this feature has no practical use, only differ between -1 and 0
+ */
+const DEFAULT_VIEWPORT_EXPANSION = -1
+
+export function resolveViewportExpansion(viewportExpansion?: number): number {
+	return viewportExpansion ?? DEFAULT_VIEWPORT_EXPANSION
+}
+
 export interface DomConfig {
+	viewportExpansion?: number
 	interactiveBlacklist?: (Element | (() => Element))[]
 	interactiveWhitelist?: (Element | (() => Element))[]
 	includeAttributes?: string[]
@@ -21,6 +36,8 @@ export interface DomConfig {
 const newElementsCache = new WeakMap<HTMLElement, string>()
 
 export function getFlatTree(config: DomConfig): FlatDomTree {
+	const viewportExpansion = resolveViewportExpansion(config.viewportExpansion)
+
 	const interactiveBlacklist = [] as Element[]
 	for (const item of config.interactiveBlacklist || []) {
 		if (typeof item === 'function') {
@@ -43,7 +60,7 @@ export function getFlatTree(config: DomConfig): FlatDomTree {
 		doHighlightElements: true,
 		debugMode: true,
 		focusHighlightIndex: -1,
-		viewportExpansion: VIEWPORT_EXPANSION,
+		viewportExpansion,
 		interactiveBlacklist,
 		interactiveWhitelist,
 		highlightOpacity: config.highlightOpacity ?? 0.0,