修复 EPUB 文本分页显示错位并补充分页调试能力

本次提交围绕 DTCoreText 文本页的分页一致性、交互索引和高亮命中进行了集中修复。

主要改动：

1. 文本页显示改为基于整章 attributed string 的上下文布局，只对当前页 range 进行渲染，避免页面子串重新换行导致的页末断行偏差。

2. 页面布局快照与交互控制器统一改为使用 chapter-absolute 索引，修正点击、选区、菜单锚点与高亮矩形在整章上下文下的定位。

3. 修复跨章节高亮串页问题，并调整文本页高亮命中逻辑：保留 CoreText 层绘制，点击时按高亮真实 rect 精确命中，避免重复绘制和整行误判。

4. 收紧 reader 级页面缓存策略，避免预加载同时持有多份整章显示副本带来的内存放大。

5. 新增分页边界校验器、垂直对齐器和 settings-flip 自动化调试入口，用于复现与诊断页范围/显示度量不一致问题。

6. 放宽 inline attachment 的 avoid-break 处理，并补充相关分页问题调查文档与索引。

Doc/PAGINATION_ISSUES_2026-07-06.md

@@ -0,0 +1,123 @@
+# 分页问题调查记录（2026-07-06）
+
+调查载体：《宝山辽墓材料与释读》（textReflowable / DTCoreText 路径），iPhone 15 Pro Max 尺寸（430×932，内容区 398×815pt）。当日共调查两个独立问题：
+
+---
+
+## 问题一：页底留空一行多，下一页首行未上移（已修复）
+
+### 现象
+
+第 6 页底部留有约 1.7 行高的空白，第 7 页首行"人目为帝羓，信有之也。[注]"本可容纳在第 6 页。截图实测：行距 75px@3x（25pt），第 6 页底部空隙 138px，足够再排一行。
+
+### 根因链
+
+1. `RDEPUBSemanticMarkerInjector.inferredHints` 给**所有** `<img>` 打上 `avoidPageBreakInside` 提示——包括行内脚注小图标（`<img class="qqreader-footnote">`，即"注"字图标）。
+2. `RDEPUBPageBreakPolicy.shouldTreatAvoidHintAsBlockProtection` 本有豁免：`blockKind == .attachment && placement != .centered → 不保护`。但 `applyPaginationSemantics` 按**结束标记的字符串顺序**应用属性：`<img>` 的结束标记先于外层 `<p>`，段落随后把图标位置的 `.rdPageBlockKind` 从 `attachment` 覆盖为 `paragraph`；而 `hints` 与 `placement` 因段落不携带这两个属性而幸存。豁免条件因此失效。
+3. `trimmedRangeForAvoidPageBreakInside` 把含注图标的行当作"不可分页块"的行，从页底裁掉（最多 3 行），推到下一页，留下空白。
+
+### 验证手段
+
+- Demo 启动参数 `--demo-pagination-debug` 输出 `[PAGINATION-DEBUG] avoidPageBreakInside removed N lines: …`；修复前被裁的行**全部**含 ``（脚注图标）。
+- 复现命令：`--demo-book-title 宝山 --demo-page 6 --demo-reset-state --demo-clear-cache --demo-pagination-debug`。
+
+### 修复
+
+`RDEPUBPageBreakPolicy.swift`：豁免条件放宽为 `(blockKind == .attachment || placement != nil) && placement != .centered`——不再依赖易被覆盖的 blockKind，只要附件 placement 是行内/基线（非居中）就不锁行。居中插图（bodyPic）的整块保护不受影响；同时覆盖 `s-pic`/`h-pic`/`g-pic` 等生僻字行内小图。
+
+修复后验证：含 `` 的裁剪从多处降为 0；"人目为帝羓"行回到第 6 页且下一段首行补齐；全书页数 280 → 273（消除欠填页）。
+
+> 备选方案（未采用）：修 `applyPaginationSemantics` 的嵌套覆盖顺序（内层优先）。会改变列表/表格的 blockRange 语义，风险大。
+
+---
+
+## 问题二：页范围与显示内容度量错配，行中断页（2026-07-07 已定位根因，见文末新增章节）
+
+### 现象
+
+第 10 页最后一行只有孤字"相"，第 11 页从"对简化，且无构造细致的翼墙。"开始——断点落在句子中间而非行边界。
+
+### 铁证推理
+
+当前设置下满行 26 字（字号 15、内容宽 398pt）。第 10 页范围要在"…而2号墓的门楼则相"后断开，要求该行装下 **27 字**——当前排版下不可能。**结论：页范围产生于另一套度量（更小字号或更宽版面），与显示时的排版不一致。** 同一套排版中分页断点必然落在行边界，孤字不可能出现。
+
+### 已排除（受控实验）
+
+在 `RDEPUBChapterLoader` 临时加 `--demo-chapter-dump` 转储（typesetString 逐属性段 + 页范围，写入 app Documents，实验后已还原）：
+
+- 冷启动（`MISS(fullRender)` 全量渲染分页）与热启动（`HIT(diskSummary)` 复用磁盘页范围）两条路径的字符串与页范围**完全一致**。字体压平、双重 `normalizeReadingAttributes`、语言码缺失（`makeChapterRenderRequest` 未传 `contentLanguageCode`）、`TailNormalizer` 尾页合并均验证为无影响或幂等。
+- 持久缓存键控正确：`RDEPUBChapterCacheKey.renderSignature` 含字体名/字号/行距倍数/lineSpacing/`layoutConfig.cacheSignature`（宽高、四边距、分栏、hyphenation、schemaVersion=17）+ 章节内容哈希。同设置跨启动复用安全。
+
+### 主要嫌疑：会话中改设置的换表过渡态
+
+- 用户两组截图之间调过行距（行距 25pt/页 32 行 → 28pt/页 28 行，字号未变）；出现问题时显示总页数 196，既非 1.6 档全量分页（~273）也非 1.8 档（~290）——当时显示的是**未收敛的中间页表**。
+- 代码窗口：
+  - `RDEPUBReaderRuntime.scheduleSettingsPreviewRepagination`：设置面板打开期间只重排**当前章**并 `applySettingsPreviewPageMap` 换部分页表，全量重排推迟到面板关闭（`needsFullRepaginationAfterSettingsClose`）。
+  - `RDEPUBChapterRuntimeStore.chapterDataCache` 仅按 spineIndex 键控（无样式维度）；`invalidateAllForSettingsChange` 清空后，**变更前已在飞行中的构建**完成时会把旧样式章节重新 `insertChapter` 插回。
+- 这些窗口里可能出现"旧页范围 + 新排版内容"的错配。
+
+### 待办 / 复现所需
+
+1. 确认触发操作：是否在设置面板调整行距/字号后（未关面板或刚关）翻页出现。
+2. 确认重启 app 后是否自愈（受控实验表明缓存路径本身一致，理应自愈；若不自愈则有持久化的脏状态）。
+3. 修复方向（待复现后定）：设置变更代（generation）标记贯穿构建流程，飞行中的旧代构建完成后丢弃、不得插回 store；或 `chapterDataCache` 键加入 renderSignature。
+
+### 经验教训
+
+- 跨启动比较"第 N 页"截图无效：章节渐进加载过程中全局页码会漂移。
+- 判断断点是否合法的快捷方法：数满行字数。断点不在行边界 ⟺ 范围与显示度量不一致。
+
+---
+
+## 相关工具与命令备忘
+
+| 用途 | 方法 |
+|---|---|
+| 分页裁剪日志 | 启动参数 `--demo-pagination-debug` |
+| 自动打开书籍/翻页 | `--demo-book-title 宝山 --demo-page N --demo-clear-cache --demo-reset-state` |
+| 注入阅读设置 | `simctl spawn <sim> defaults write cn.shen.ReadViewDemo ssreader.epub.settings -data <hex(JSON)>`，JSON 形如 `{"fontSize":15,"lineHeightMultiple":1.8}` |
+| 截屏 | `xcrun simctl io <sim> screenshot <绝对路径>`（相对路径会因只读文件系统失败） |
+| 模拟器 | 预装仅 16/17 系列；`simctl create` 可建 iPhone 15 Pro Max（430×932@3x 与问题截图同尺寸） |
+| 页边界/换行一致性校验 | 启动参数 `--demo-pagination-validate`（RDEPUBTextPageBoundaryValidator，逐页比对显示换行与全章上下文换行，输出 STALE-RANGE / DISPLAY-DIVERGE / DIAGNOSE / ATTR-DIFF） |
+| 设置面板自动翻转 | 启动参数 `--demo-settings-flip`（RDEPUBSettingsFlipAutomation，自动开面板→改行距→关面板→翻页，三种时序） |
+
+---
+
+## 问题二根因（2026-07-07 续查确认）
+
+### 结论
+
+**分页与显示使用同一引擎（DTCoreText/CoreText）但输入字符串不同：分页在“全章字符串”上下文中断行；显示把页内容取成子串后重新断行。`CTTypesetterSuggestLineBreak` 在同一文字、同一属性、同一宽度下，会因字符串上下文不同而在 CJK 标点压缩/悬挂处产生 ±1 字的断点漂移。** 页范围（全章上下文产物）与显示换行（子串产物）在页内任何一行发生漂移，累积到页尾就出现孤字（用户看到的「相」）或半空行。
+
+与缓存、设置换表竞态无关：设置变更只是**搬动了页边界位置**，让某个边界恰好落在漂移点上，症状因此看似随设置变化出现/消失。这同时解释了此前所有观察：冷/热启动转储完全一致（分页自身是一致的，错配发生在分页 vs 显示）；重启后"第 N 页"不复现（边界挪走了）。
+
+### 证据（宝山书 spine 3，字号 15 / 行距 1.6 / 宽 398pt）
+
+`--demo-pagination-validate` 基线运行（无任何设置操作）即命中，前 8 页中 5 页分叉。决定性样本 page 8/55 第 11 行（行首 offset 4224）：
+
+- 显示端断点 4252（行尾"…可以相"——孤字机制当场复现），全章上下文断点 4253（"…可以相当"）
+- 逐字符属性 diff：**完全一致**（无 ATTR-DIFF）
+- 对照排版：原始子串 = 4252，段落级上下文 = 4252，**只有全章上下文 = 4253**
+- page 6/55 的上下文探针行宽 408.39 > 框宽 398 —— 标点悬挂/压缩越界的直接证据
+
+即断点差异既不是首行缩进归一化（`normalizedPageContent` 的 firstLineHeadIndent 处理本身是正确的），也不是属性差异，而是 CoreText 断行对字符串上下文（跨段落！）敏感。
+
+### 修复（方案 A，2026-07-07 已实施）
+
+显示端改为**全章上下文排版**：`RDEPUBTextContentView.configure` 取 `page.chapterContent` 的整章副本作为显示内容，`DTCoreTextLayouter(章节副本).layoutFrame(bounds, range: page.contentRange)` 只排当页范围。断行与分页器按构造一致（同串、同起点、同宽）。
+
+配套改动：
+
+- layoutFrame 的 stringRange 因此为**章节绝对坐标**。`RDEPUBPageLayoutSnapshot.build` 去掉 +pageStartOffset 归一（本就输出绝对坐标给消费方），`RDEPUBPageInteractionController` 去掉全部 -pageOffset 反换算；选区/高亮/点击测试/装饰层的消费接口本来就是绝对坐标，无需变动。
+- 主题前景色、暗色图片调整（`RDEPUBDarkImageAdjuster` 新增 `in range:` 参数）、高亮标记只施加在章节副本的当页范围上；`applyHighlightsToContent` 改用绝对 overlap 范围。
+- 删除 DTCoreText 路径对 `normalizedPageContent` 的调用——续段首行缩进/段前距归一化 hack 由上下文排版天然取代（非 DTCoreText 回退路径仍保留）。
+- layouter（framesetter）随显示内容缓存于 cell（`coreTextLayouter`），bounds 变化只重建 layoutFrame，避免每次 layout pass 对整章重建 framesetter。
+- `RDEPUBTextPageBoundaryValidator` 适配绝对坐标，保留为回归警报。
+
+验证：`--demo-pagination-validate` 基线（原先 spine 3 前 8 页 5 处分叉）修复后 **0 命中**；第 10 页首行断点与分页一致（「…使用了更多」）；Selection/Annotation UI 测试回归通过（见下）。
+
+### 遗留（独立的潜在缺陷，本次未触发）
+
+- `RDEPUBChapterRuntimeStore.chapterDataCache` 仅按 spineIndex 键控；设置变更时飞行中的旧构建完成后仍会插回（`RDEPUBChapterLoader.loadChapter` 的 `insertChapter` 无代际校验）。
+- `RDEPUBChapterLoader.pendingLoads` 按 spineIndex 合流，跨设置变更合流会把旧样式章节交给新请求。
+- 建议修复问题二后用 `--demo-settings-flip` + `--demo-pagination-validate` 回归验证这两个窗口。

Doc/index.md

@@ -37,6 +37,7 @@
 | [CHAPTER_RUNTIME.md](CHAPTER_RUNTIME.md) | 章节运行时详解：按需加载、章节窗口协调、页图管理、磁盘缓存、后台补全 |
 | [CFI_SUBSYSTEM.md](CFI_SUBSYSTEM.md) | CFI 子系统详解：EPUB CFI 解析、生成、序列化、范围、恢复引擎 |
 | [CFI_ISSUES_REVIEW.md](CFI_ISSUES_REVIEW.md) | CFI 实现问题分析报告：11 个问题（3 高 / 5 中 / 3 低），含修复优先级建议 |
+| [PAGINATION_ISSUES_2026-07-06.md](PAGINATION_ISSUES_2026-07-06.md) | 分页问题调查记录：脚注图标 avoid-trim 裁行（已修复）＋页范围/显示度量错配行中断页（待复现），含调试命令备忘 |
 | [API_REFERENCE.md](API_REFERENCE.md) | 公开 API 参考：RDEPUBReaderController 公开接口、委托协议、配置模型 |
 | [当前阅读器问题修复开发清单.md](当前阅读器问题修复开发清单.md) | 基于当前代码实现整理的逐任务开发计划：10 个任务、修改位置、实施步骤、风险点与验收标准 |
 | [大书远距目录跳转与后台补全优化方案.md](大书远距目录跳转与后台补全优化方案.md) | 面向大书按需分页模式的完整优化方案：远距目录跳转、后台补全优先级、页图接管协议与分阶段实施路径 |

Sources/RDReaderView/EPUBTextRendering/Pagination/RDEPUBPageBreakPolicy.swift

@@ -188,7 +188,12 @@ struct RDEPUBPageBreakPolicy {
         let blockKind = (attributes[.rdPageBlockKind] as? String)
             .flatMap(RDEPUBTextBlockKind.init(rawValue:))
 
-        if blockKind == .attachment, placement != .centered {
+        // Inline attachments (footnote icons, rare-character images) flow with
+        // the surrounding text, so their avoid hint must not lock the line to
+        // the next page. Placement is checked besides blockKind because an
+        // enclosing paragraph's semantics overwrite blockKind on the
+        // attachment's range, while placement survives.
+        if blockKind == .attachment || placement != nil, placement != .centered {
             return false
         }
 

Sources/RDReaderView/EPUBUI/RDEPUBReaderController.swift

@@ -281,6 +281,7 @@ public final class RDEPUBReaderController: UIViewController {
     public override func viewDidAppear(_ animated: Bool) {
         super.viewDidAppear(animated)
         startInitialLoadIfNeeded()
+        RDEPUBSettingsFlipAutomation.startIfNeeded(controller: self)
     }
 
     public override func viewDidLayoutSubviews() {

Sources/RDReaderView/EPUBUI/RDEPUBSettingsFlipAutomation.swift

@@ -0,0 +1,84 @@
+import UIKit
+
+/// Debug-only automation (`--demo-settings-flip`) that replays the user
+/// gesture sequence suspected of producing stale page tables: open the
+/// settings panel, change the line height, close the panel, flip pages —
+/// with several timing variants aimed at the preview-repagination and
+/// in-flight chapter build races. Combine with
+/// `--demo-pagination-validate` to detect any resulting metric mismatch.
+enum RDEPUBSettingsFlipAutomation {
+
+    static let isEnabled = ProcessInfo.processInfo.arguments.contains("--demo-settings-flip")
+
+    private(set) static var hasStarted = false
+
+    static func startIfNeeded(controller: RDEPUBReaderController) {
+        guard isEnabled, !hasStarted else { return }
+        hasStarted = true
+        print("[SETTINGS-FLIP] scheduled")
+
+        // Pass 1 starts while progressive pagination of the freshly opened
+        // book is still running, so prefetch builds are in flight.
+        run(after: 2.0) { [weak controller] in
+            guard let controller else { return }
+            pass(controller: controller, index: 1, lineHeight: 1.8, changeToCloseDelay: 1.2) {
+                pass(controller: controller, index: 2, lineHeight: 1.6, changeToCloseDelay: 0.05) {
+                    pass(controller: controller, index: 3, lineHeight: 1.8, changeToCloseDelay: 0.3) {
+                        print("[SETTINGS-FLIP] finished all passes")
+                    }
+                }
+            }
+        }
+    }
+
+    /// One panel round-trip: present → change line height → close after
+    /// `changeToCloseDelay` → flip pages forward and back.
+    private static func pass(
+        controller: RDEPUBReaderController,
+        index: Int,
+        lineHeight: CGFloat,
+        changeToCloseDelay: TimeInterval,
+        completion: @escaping () -> Void
+    ) {
+        print("[SETTINGS-FLIP] pass \(index) present panel")
+        controller.presentSettings()
+
+        run(after: 0.7) { [weak controller] in
+            guard let controller else { return }
+            print("[SETTINGS-FLIP] pass \(index) set lineHeightMultiple=\(lineHeight)")
+            controller.updateConfiguration { $0.lineHeightMultiple = lineHeight }
+
+            run(after: changeToCloseDelay) { [weak controller] in
+                guard let controller else { return }
+                print("[SETTINGS-FLIP] pass \(index) close panel")
+                controller.dismiss(animated: true) { [weak controller] in
+                    controller?.runtime.settingsPanelDidDisappear()
+                    run(after: 1.0) { [weak controller] in
+                        guard let controller else { return }
+                        flipPages(controller: controller, index: index, completion: completion)
+                    }
+                }
+            }
+        }
+    }
+
+    private static func flipPages(
+        controller: RDEPUBReaderController,
+        index: Int,
+        completion: @escaping () -> Void
+    ) {
+        let startPage = controller.readerView.currentPage + 1
+        let offsets = [1, 2, 3, 2, 1, 0]
+        print("[SETTINGS-FLIP] pass \(index) flip pages from \(startPage)")
+        for (step, offset) in offsets.enumerated() {
+            run(after: 0.6 * Double(step + 1)) { [weak controller] in
+                _ = controller?.go(toPageNumber: startPage + offset, animated: false)
+            }
+        }
+        run(after: 0.6 * Double(offsets.count + 1) + 0.5, block: completion)
+    }
+
+    private static func run(after delay: TimeInterval, block: @escaping () -> Void) {
+        DispatchQueue.main.asyncAfter(deadline: .now() + delay, execute: block)
+    }
+}

Sources/RDReaderView/EPUBUI/TextPage/RDEPUBDarkImageAdjuster.swift

@@ -25,6 +25,7 @@ enum RDEPUBDarkImageAdjuster {
 
     static func adjustIfNeeded(
         _ content: NSMutableAttributedString,
+        in range: NSRange? = nil,
         configuration: RDEPUBReaderConfiguration
     ) -> NSMutableAttributedString {
         guard configuration.darkImageAdjustmentEnabled,
@@ -34,7 +35,8 @@ enum RDEPUBDarkImageAdjuster {
         }
 
         let fullRange = NSRange(location: 0, length: content.length)
-        content.enumerateAttribute(.attachment, in: fullRange) { value, range, _ in
+        let targetRange = range.map { NSIntersectionRange($0, fullRange) } ?? fullRange
+        content.enumerateAttribute(.attachment, in: targetRange) { value, range, _ in
             guard let attachment = value as? DTImageTextAttachment,
                   !isCoverAttachment(attachment),
                   let image = attachment.image,

Sources/RDReaderView/EPUBUI/TextPage/RDEPUBPageInteractionController.swift

@@ -44,10 +44,12 @@ final class RDEPUBPageInteractionController {
             x: point.x - line.baselineOrigin.x,
             y: point.y - line.baselineOrigin.y
         )
+        // The layout frame is built in chapter context, so DTCoreText string
+        // indices are chapter-absolute already.
         let idx = dtLine.stringIndex(forPosition: relativePoint)
         guard idx != NSNotFound, idx >= 0 else { return nil }
         return normalizedIndex(
-            idx + pageOffset(for: snapshot),
+            idx,
             lineRange: line.stringRange,
             pageRange: snapshot.pageContentRange
         )
@@ -74,10 +76,9 @@ final class RDEPUBPageInteractionController {
 
 #if canImport(DTCoreText)
             guard let dtLine = dtLineContaining(range: line.stringRange) else { continue }
-            let pageOffset = pageOffset(for: snapshot)
+            let startX = dtLine.offset(forStringIndex: overlap.location)
-            let startX = dtLine.offset(forStringIndex: overlap.location - pageOffset)
             let endIdx = overlap.location + overlap.length
-            let endX = dtLine.offset(forStringIndex: endIdx - pageOffset)
+            let endX = dtLine.offset(forStringIndex: endIdx)
 #else
             let startX: CGFloat = 0
             let endX: CGFloat = line.frame.width
@@ -133,7 +134,7 @@ final class RDEPUBPageInteractionController {
 
 #if canImport(DTCoreText)
         guard let dtLine = dtLineContaining(range: line.stringRange) else { return nil }
-        let offsetX = dtLine.offset(forStringIndex: index - pageOffset(for: snapshot))
+        let offsetX = dtLine.offset(forStringIndex: index)
 #else
         let offsetX: CGFloat = 0
 #endif
@@ -183,16 +184,11 @@ final class RDEPUBPageInteractionController {
 #if canImport(DTCoreText)
 
     private func dtLineContaining(range: NSRange) -> DTCoreTextLayoutLine? {
-        guard let dtLayoutFrame, let snapshot else { return nil }
+        guard let dtLayoutFrame else { return nil }
-        let localLocation = max(range.location - pageOffset(for: snapshot), 0)
+        return dtLayoutFrame.lineContaining(UInt(max(range.location, 0)))
-        return dtLayoutFrame.lineContaining(UInt(localLocation))
     }
 #endif
 
-    private func pageOffset(for snapshot: RDEPUBPageLayoutSnapshot) -> Int {
-        snapshot.page.pageStartOffset
-    }
-
     private func mergeAdjacentRects(_ rects: [CGRect]) -> [CGRect] {
         guard rects.count > 1 else { return rects }
 

Sources/RDReaderView/EPUBUI/TextPage/RDEPUBPageLayoutSnapshot.swift

@@ -130,13 +130,14 @@ struct RDEPUBPageLayoutSnapshot {
             return nil
         }
 
+        // The layout frame is produced in chapter context, so DTCoreText
+        // string ranges are already chapter-absolute.
         var lines: [RDEPUBPageLine] = []
         var runs: [RDEPUBPageRun] = []
         var attachments: [RDEPUBPageAttachment] = []
-        let pageOffset = page.pageStartOffset
 
         for dtLine in dtLines {
-            let lineRange = offset(dtLine.stringRange(), by: pageOffset)
+            let lineRange = dtLine.stringRange()
             let line = RDEPUBPageLine(
                 stringRange: lineRange,
                 frame: dtLine.frame,
@@ -149,7 +150,7 @@ struct RDEPUBPageLayoutSnapshot {
 
             if let glyphRuns = dtLine.glyphRuns as? [DTCoreTextGlyphRun] {
                 for run in glyphRuns {
-                    let runRange = offset(run.stringRange(), by: pageOffset)
+                    let runRange = run.stringRange()
                     let isAttachment = run.attachment != nil
                     runs.append(
                         RDEPUBPageRun(
@@ -177,7 +178,7 @@ struct RDEPUBPageLayoutSnapshot {
             }
         }
 
-        let visibleRange = offset(layoutFrame.visibleStringRange(), by: pageOffset)
+        let visibleRange = layoutFrame.visibleStringRange()
 
         return RDEPUBPageLayoutSnapshot(
             page: page,
@@ -219,11 +220,5 @@ struct RDEPUBPageLayoutSnapshot {
         return RDEPUBAttachmentNormalizer.attachmentKind(for: attributes)
     }
 
-    private static func offset(_ range: NSRange, by offset: Int) -> NSRange {
-        guard range.location != NSNotFound else {
-            return range
-        }
-        return NSRange(location: range.location + offset, length: range.length)
-    }
 #endif
 }

Sources/RDReaderView/EPUBUI/TextPage/RDEPUBTextContentView.swift

@@ -90,6 +90,10 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate, RDReader
     private var coreTextDisplayContent: NSAttributedString?
 
     private var coreTextDisplayRange: NSRange?
+
+    /// Framesetter over the full chapter copy, rebuilt when the display
+    /// content changes; layout frames are recomputed per bounds change.
+    private var coreTextLayouter: DTCoreTextLayouter?
 #endif
 
     private let interactionController = RDEPUBPageInteractionController()
@@ -313,7 +317,18 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate, RDReader
     }
 
     var shouldAvoidReaderPageCaching: Bool {
-        currentPage == nil || loadingSpinner.isAnimating
+#if canImport(DTCoreText)
+        // In-context text layout keeps a chapter-length attributed copy and
+        // layouter alive for the visible page. Avoid reader-level page
+        // caching so preloading does not retain several full-chapter
+        // display copies at once.
+        return currentPage == nil
+            || loadingSpinner.isAnimating
+            || coreTextDisplayContent != nil
+#else
+        currentPage == nil
+            || loadingSpinner.isAnimating
+#endif
     }
 
     private var hasInteractiveTextContent: Bool {
@@ -405,6 +420,7 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate, RDReader
             coreTextContentView.layoutFrame = nil
             coreTextDisplayContent = nil
             coreTextDisplayRange = nil
+            coreTextLayouter = nil
 #endif
             updateStaticGestureAvailability()
             updateSelectionPanAvailability()
@@ -418,15 +434,26 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate, RDReader
         coverImageView.image = nil
 
 #if canImport(DTCoreText)
-        let displayContent = RDEPUBDarkImageAdjuster.adjustIfNeeded(
+        // In-context display: keep the full chapter string and lay out only the
-            normalizedPageContent(from: page),
+        // page's range. CTTypesetter line breaks are context-sensitive, so
+        // re-wrapping a page substring can break ±1 character away from the
+        // paginator's line ends (lone characters spilling onto the last line);
+        // laying out the same chapter string the paginator used cannot.
+        // String indices in the layout frame are therefore chapter-absolute.
+        let displayContent = NSMutableAttributedString(attributedString: page.chapterContent)
+        let pageRange = NSIntersectionRange(
+            page.contentRange,
+            NSRange(location: 0, length: displayContent.length)
+        )
+        _ = RDEPUBDarkImageAdjuster.adjustIfNeeded(
+            displayContent,
+            in: pageRange,
             configuration: configuration
         )
-        let fullRange = NSRange(location: 0, length: displayContent.length)
         displayContent.addAttribute(
             .foregroundColor,
             value: configuration.theme.contentTextColor,
-            range: fullRange
+            range: pageRange
         )
 
         applyHighlightsToContent(displayContent, highlights: highlights, page: page)
@@ -434,7 +461,9 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate, RDReader
         coreTextContentView.backgroundColor = .clear
         coreTextContentView.accessibilityIdentifier = "epub.reader.selection.text"
         coreTextDisplayContent = displayContent
-        coreTextDisplayRange = NSRange(location: 0, length: displayContent.length)
+        coreTextDisplayRange = pageRange
+        coreTextLayouter = DTCoreTextLayouter(attributedString: displayContent)
+        coreTextLayouter?.shouldCacheLayoutFrames = false
         coreTextContentView.attributedDisplayContent = displayContent
         updateCoreTextLayoutFrameIfNeeded()
 #else
@@ -493,6 +522,7 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate, RDReader
         coreTextContentView.layoutFrame = nil
         coreTextDisplayContent = nil
         coreTextDisplayRange = nil
+        coreTextLayouter = nil
 #endif
 
         overlayView.clearSelection()
@@ -540,6 +570,7 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate, RDReader
         coreTextContentView.layoutFrame = nil
         coreTextDisplayContent = nil
         coreTextDisplayRange = nil
+        coreTextLayouter = nil
 #endif
         return true
     }
@@ -581,26 +612,27 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate, RDReader
         return image(from: attachmentValue)
     }
 
+    /// Marks highlight ranges on the chapter-length display copy. Ranges stay
+    /// chapter-absolute — the in-context layout frame uses the same indices.
     private func applyHighlightsToContent(
         _ content: NSMutableAttributedString,
         highlights: [RDEPUBHighlight],
         page: RDEPUBTextPage
     ) {
         for highlight in highlights {
+            guard highlight.location.href == page.href else { continue }
             guard let rangeInfo = highlight.rangeInfo,
                   let info = RDEPUBTextOffsetRangeInfo.decode(from: rangeInfo),
                   let absoluteRange = info.nsRange else { continue }
             let overlap = NSIntersectionRange(absoluteRange, page.contentRange)
-            guard overlap.length > 0 else { continue }
+            guard overlap.length > 0,
-            let relativeRange = NSRange(location: overlap.location - page.pageStartOffset, length: overlap.length)
+                  overlap.location + overlap.length <= content.length else { continue }
-            guard relativeRange.location >= 0,
-                  relativeRange.location + relativeRange.length <= content.length else { continue }
 
             switch highlight.style {
             case .highlight:
-                content.addAttribute(kRDEPUBHighlightAttributeName, value: highlight.uiColor, range: relativeRange)
+                content.addAttribute(kRDEPUBHighlightAttributeName, value: highlight.uiColor, range: overlap)
             case .underline:
-                content.addAttribute(kRDEPUBUnderlineAttributeName, value: highlight.uiColor, range: relativeRange)
+                content.addAttribute(kRDEPUBUnderlineAttributeName, value: highlight.uiColor, range: overlap)
             }
         }
     }
@@ -698,13 +730,26 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate, RDReader
             return
         }
 
-        guard let layouter = DTCoreTextLayouter(attributedString: displayContent) else {
+        guard let layouter = coreTextLayouter else {
             coreTextContentView.layoutFrame = nil
             interactionController.configure(layoutFrame: nil, page: page)
             return
         }
-        layouter.shouldCacheLayoutFrames = false
         let layoutFrame = layouter.layoutFrame(with: coreTextContentView.bounds, range: displayRange)
+        if let layoutFrame {
+            RDEPUBTextPageBoundaryValidator.validate(
+                page: page,
+                displayLayoutFrame: layoutFrame,
+                displayContent: displayContent,
+                displayBounds: coreTextContentView.bounds
+            )
+            RDEPUBTextPageVerticalJustifier.justify(
+                layoutFrame,
+                contentHeight: coreTextContentView.bounds.height,
+                isChapterLastPage: page.pageIndexInChapter >= page.totalPagesInChapter - 1,
+                pixelScale: window?.screen.scale ?? UIScreen.main.scale
+            )
+        }
         coreTextContentView.layoutFrame = layoutFrame
         interactionController.configure(layoutFrame: layoutFrame, page: page)
         overlayView.updateSnapshot(interactionController.snapshot)
@@ -713,6 +758,9 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate, RDReader
         if let page = currentPage {
             let (bgDecorations, fgDecorations) = overlayView.buildDecorations(
                 page: page,
+                // Text-page highlights/underlines are already painted by the
+                // CoreText render view via display-content attributes. Keep
+                // overlay decorations for search only to avoid double drawing.
                 highlights: [],
                 searchState: currentSearchState,
                 interactionController: interactionController
@@ -845,10 +893,15 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate, RDReader
               !targetRect.isEmpty else {
             return
         }
+        let resolvedTargetRect = resolvedSelectionMenuTargetRect(from: targetRect)
         if #available(iOS 16.0, *), let interaction = _editMenuInteraction as? UIEditMenuInteraction {
-            let anchor = CGPoint(x: targetRect.midX, y: targetRect.midY)
+            becomeFirstResponder()
+            let anchor = CGPoint(x: resolvedTargetRect.midX, y: resolvedTargetRect.midY)
             let config = UIEditMenuConfiguration(identifier: "SelectionMenu", sourcePoint: anchor)
-            interaction.presentEditMenu(with: config)
+            DispatchQueue.main.async { [weak self, weak interaction] in
+                guard let self, self.currentSelection != nil else { return }
+                interaction?.presentEditMenu(with: config)
+            }
         } else {
             becomeFirstResponder()
             let menuController = UIMenuController.shared
@@ -857,7 +910,7 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate, RDReader
                 UIMenuItem(title: "高亮", action: #selector(rd_highlight(_:))),
                 UIMenuItem(title: "批注", action: #selector(rd_annotate(_:)))
             ]
-            menuController.setTargetRect(targetRect, in: coreTextRenderView ?? self)
+            menuController.setTargetRect(resolvedTargetRect, in: self)
             menuController.setMenuVisible(true, animated: true)
         }
     }
@@ -891,6 +944,11 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate, RDReader
 #endif
     }
 
+    private func resolvedSelectionMenuTargetRect(from rect: CGRect) -> CGRect {
+        guard let renderView = coreTextRenderView else { return rect }
+        return convert(rect, from: renderView)
+    }
+
     private func updateAccessibilityDecorationSummary() {
 #if canImport(DTCoreText)
         coreTextContentView.accessibilityValue = [
@@ -901,14 +959,17 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate, RDReader
     }
 
     private func highlight(at point: CGPoint) -> RDEPUBHighlight? {
-        guard currentPage != nil else { return nil }
+        guard let page = currentPage else { return nil }
-        let absoluteRange = backgroundOverlayView.absoluteRange(at: point) ?? overlayView.absoluteRange(at: point)
-        guard let absoluteRange else { return nil }
         let matches = currentHighlights.filter { highlight in
-            guard let range = RDEPUBTextOffsetRangeInfo.decode(from: highlight.rangeInfo)?.nsRange else {
+            guard highlight.location.href == page.href,
+                  let range = RDEPUBTextOffsetRangeInfo.decode(from: highlight.rangeInfo)?.nsRange else {
                 return false
             }
-            return NSIntersectionRange(range, absoluteRange).length > 0
+            let overlap = NSIntersectionRange(range, page.contentRange)
+            guard overlap.length > 0 else { return false }
+            return interactionController.selectionRects(for: overlap).contains {
+                $0.insetBy(dx: -4, dy: -4).contains(point)
+            }
         }
         return matches.sorted { lhs, rhs in
             let lhsRange = RDEPUBTextOffsetRangeInfo.decode(from: lhs.rangeInfo)?.nsRange?.length ?? .max
@@ -1090,8 +1151,9 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate, RDReader
 extension RDEPUBTextContentView: UIEditMenuInteractionDelegate {
     func editMenuInteraction(
         _ interaction: UIEditMenuInteraction,
-        menuFor configuration: UIEditMenuConfiguration
+        menuFor configuration: UIEditMenuConfiguration,
-    ) -> UIMenu {
+        suggestedActions: [UIMenuElement]
+    ) -> UIMenu? {
         UIMenu(children: [
             UICommand(title: "拷贝", action: #selector(rd_copy(_:))),
             UICommand(title: "高亮", action: #selector(rd_highlight(_:))),
@@ -1105,7 +1167,7 @@ extension RDEPUBTextContentView: UIEditMenuInteractionDelegate {
     ) -> CGRect {
         if let targetRect = selectionController.menuAnchorRect(interactionController: interactionController),
            !targetRect.isEmpty {
-            return targetRect
+            return resolvedSelectionMenuTargetRect(from: targetRect)
         }
         return bounds
     }

Sources/RDReaderView/EPUBUI/TextPage/RDEPUBTextPageBoundaryValidator.swift

@@ -0,0 +1,223 @@
+import UIKit
+
+#if canImport(DTCoreText)
+import DTCoreText
+
+/// Debug-only detector for pagination/display metric mismatches, enabled by
+/// the `--demo-pagination-validate` launch argument.
+///
+/// For every displayed text page it re-wraps the chapter text from the page
+/// start at the display width (same CoreText engine the paginator used, in
+/// chapter context) and compares the resulting line breaks against both the
+/// page range and the lines actually drawn. Two failure classes:
+///
+/// - `STALE-RANGE`: the page range does not end on a line boundary of the
+///   current metrics — the range was produced under different metrics than
+///   the ones on screen (stale page table).
+/// - `DISPLAY-DIVERGE`: the range is fine, but the drawn lines break at
+///   different offsets than the in-context wrap — the display-side content
+///   transform (e.g. continuation-paragraph normalization) changed wrapping.
+enum RDEPUBTextPageBoundaryValidator {
+
+    static let isEnabled = ProcessInfo.processInfo.arguments.contains("--demo-pagination-validate")
+
+    /// Extra characters wrapped past the page end so the probe can see the
+    /// line that a mid-line page boundary cuts through.
+    private static let probeTailLength = 400
+
+    static func validate(
+        page: RDEPUBTextPage,
+        displayLayoutFrame: DTCoreTextLayoutFrame,
+        displayContent: NSAttributedString?,
+        displayBounds: CGRect
+    ) {
+        guard isEnabled else { return }
+        let chapter = page.chapterContent
+        let pageStart = page.contentRange.location
+        let pageEnd = page.contentRange.location + page.contentRange.length
+        guard page.contentRange.length > 0,
+              pageStart >= 0,
+              pageEnd <= chapter.length,
+              displayBounds.width > 0 else { return }
+
+        guard let layouter = DTCoreTextLayouter(attributedString: chapter) else { return }
+        layouter.shouldCacheLayoutFrames = false
+        let probeLength = min(chapter.length - pageStart, page.contentRange.length + probeTailLength)
+        let probeRect = CGRect(x: 0, y: 0, width: displayBounds.width, height: 4_000_000)
+        guard let probeFrame = layouter.layoutFrame(
+            with: probeRect,
+            range: NSRange(location: pageStart, length: probeLength)
+        ), let probeLines = probeFrame.lines as? [DTCoreTextLayoutLine] else { return }
+
+        let probeRanges = probeLines.map { $0.stringRange() }
+
+        // Class A: the page must end on a line boundary of the current wrap
+        // (unless it is the chapter's last page, which ends at chapter end).
+        let isChapterLastPage = pageEnd >= chapter.length
+        if !isChapterLastPage,
+           !probeRanges.contains(where: { NSMaxRange($0) == pageEnd }),
+           let cutLine = probeRanges.first(where: { NSLocationInRange(pageEnd - 1, $0) }) {
+            let text = chapter.string as NSString
+            let lineText = safeSubstring(text, cutLine)
+            print("[PAGINATION-VALIDATE] STALE-RANGE spine=\(page.spineIndex) page=\(page.pageIndexInChapter + 1)/\(page.totalPagesInChapter) pageEnd=\(pageEnd) cutLine=\(NSStringFromRange(cutLine)) width=\(displayBounds.width) line=\"\(lineText)\"")
+        }
+
+        // Class B: the drawn lines must break at the same offsets as the
+        // in-context wrap. The display layout frame is built in chapter
+        // context, so its string ranges are chapter-absolute.
+        guard let displayLines = displayLayoutFrame.lines as? [DTCoreTextLayoutLine] else { return }
+        for (index, displayLine) in displayLines.enumerated() {
+            let displayRange = displayLine.stringRange()
+            let displayEndInChapter = NSMaxRange(displayRange)
+            guard displayEndInChapter < pageEnd else { break }
+            guard index < probeRanges.count else { break }
+            let probeEnd = NSMaxRange(probeRanges[index])
+            if probeEnd != displayEndInChapter {
+                let text = chapter.string as NSString
+                let lineStartInChapter = displayRange.location
+                let displayLineRangeInChapter = displayRange
+                let isParagraphStart = lineStartInChapter == 0
+                    || text.character(at: lineStartInChapter - 1) == 0x0A
+                let chapterStyle = chapter.attribute(
+                    .paragraphStyle, at: lineStartInChapter, effectiveRange: nil
+                ) as? NSParagraphStyle
+                let displayStyle = displayContent?.attribute(
+                    .paragraphStyle, at: displayRange.location, effectiveRange: nil
+                ) as? NSParagraphStyle
+                let displayLineWidth = displayLine.frame.width
+                let probeLineWidth = index < probeLines.count ? probeLines[index].frame.width : -1
+                print("[PAGINATION-VALIDATE] DISPLAY-DIVERGE spine=\(page.spineIndex) page=\(page.pageIndexInChapter + 1)/\(page.totalPagesInChapter) lineIndex=\(index) displayLineEnd=\(displayEndInChapter) probeLineEnd=\(probeEnd) width=\(displayBounds.width) paraStart=\(isParagraphStart) chapterIndents=(\(chapterStyle?.firstLineHeadIndent ?? -1),\(chapterStyle?.headIndent ?? -1),tail:\(chapterStyle?.tailIndent ?? -1)) displayIndents=(\(displayStyle?.firstLineHeadIndent ?? -1),\(displayStyle?.headIndent ?? -1),tail:\(displayStyle?.tailIndent ?? -1)) displayLineWidth=\(displayLineWidth) probeLineWidth=\(probeLineWidth) displayLine=\"\(safeSubstring(text, displayLineRangeInChapter))\" displayBreak=\"…\(safeSubstring(text, NSRange(location: max(displayEndInChapter - 2, 0), length: min(4, text.length - max(displayEndInChapter - 2, 0)))))\" probeBreak=\"…\(safeSubstring(text, NSRange(location: max(probeEnd - 2, 0), length: min(4, text.length - max(probeEnd - 2, 0)))))\"")
+                diagnoseDivergence(
+                    page: page,
+                    displayContent: displayContent,
+                    lineIndex: index,
+                    lineStartInChapter: lineStartInChapter,
+                    displayEndInChapter: displayEndInChapter,
+                    probeEnd: probeEnd,
+                    width: displayBounds.width
+                )
+                break
+            }
+        }
+    }
+
+    /// Narrows a display/probe line-break divergence down to its cause by
+    /// re-wrapping controlled variants and diffing attributes over the line.
+    private static func diagnoseDivergence(
+        page: RDEPUBTextPage,
+        displayContent: NSAttributedString?,
+        lineIndex: Int,
+        lineStartInChapter: Int,
+        displayEndInChapter: Int,
+        probeEnd: Int,
+        width: CGFloat
+    ) {
+        let chapter = page.chapterContent
+
+        // Variant 1: the raw page substring with no display normalization.
+        let rawSubstring = chapter.attributedSubstring(from: page.contentRange)
+        let rawEnd = lineEnd(
+            wrapping: rawSubstring,
+            lineIndex: lineIndex,
+            width: width
+        ).map { $0 + page.pageStartOffset }
+
+        // Variant 2: wrap the chapter from the start of the paragraph that
+        // contains the diverging line (context = current paragraph only).
+        let text = chapter.string as NSString
+        let paragraphRange = text.paragraphRange(
+            for: NSRange(location: lineStartInChapter, length: 0)
+        )
+        let paraString = chapter.attributedSubstring(
+            from: NSRange(
+                location: paragraphRange.location,
+                length: min(chapter.length - paragraphRange.location, paragraphRange.length + probeTailLength)
+            )
+        )
+        var paraEnd: Int?
+        if let layouter = DTCoreTextLayouter(attributedString: paraString) {
+            layouter.shouldCacheLayoutFrames = false
+            let frame = layouter.layoutFrame(
+                with: CGRect(x: 0, y: 0, width: width, height: 4_000_000),
+                range: NSRange(location: 0, length: paraString.length)
+            )
+            if let lines = frame?.lines as? [DTCoreTextLayoutLine] {
+                let target = lineStartInChapter - paragraphRange.location
+                if let matched = lines.first(where: { $0.stringRange().location == target }) {
+                    paraEnd = NSMaxRange(matched.stringRange()) + paragraphRange.location
+                }
+            }
+        }
+
+        print("[PAGINATION-VALIDATE] DIAGNOSE lineStart=\(lineStartInChapter) display=\(displayEndInChapter) probeFullContext=\(probeEnd) rawSubstringWrap=\(rawEnd ?? -1) paragraphContextWrap=\(paraEnd ?? -1)")
+
+        // Attribute diff between chapter text and display content over the
+        // diverging line (through the longer of the two ends). The display
+        // content is a chapter-length copy, so indices are shared.
+        guard let displayContent else { return }
+        let diffEnd = max(displayEndInChapter, probeEnd)
+        var position = lineStartInChapter
+        while position < diffEnd {
+            guard position >= 0, position < displayContent.length, position < chapter.length else { break }
+            var chapterRunRange = NSRange()
+            let chapterAttrs = chapter.attributes(at: position, effectiveRange: &chapterRunRange)
+            var displayRunRange = NSRange()
+            let displayAttrs = displayContent.attributes(at: position, effectiveRange: &displayRunRange)
+
+            let keys = Set(chapterAttrs.keys).union(displayAttrs.keys)
+            for key in keys {
+                let lhs = chapterAttrs[key] as AnyObject?
+                let rhs = displayAttrs[key] as AnyObject?
+                if let lhs, let rhs, lhs.isEqual(rhs) { continue }
+                if lhs == nil, rhs == nil { continue }
+                print("[PAGINATION-VALIDATE] ATTR-DIFF pos=\(position) key=\(key.rawValue) chapter=\(describeAttr(lhs)) display=\(describeAttr(rhs))")
+            }
+            let nextPosition = min(
+                NSMaxRange(chapterRunRange),
+                NSMaxRange(displayRunRange)
+            )
+            guard nextPosition > position else { break }
+            position = nextPosition
+        }
+    }
+
+    private static func describeAttr(_ value: AnyObject?) -> String {
+        guard let value else { return "nil" }
+        if let font = value as? UIFont {
+            return "font(\(font.fontName),\(font.pointSize))"
+        }
+        if let style = value as? NSParagraphStyle {
+            return "para(fli:\(style.firstLineHeadIndent),hi:\(style.headIndent),ti:\(style.tailIndent),lbm:\(style.lineBreakMode.rawValue),align:\(style.alignment.rawValue),lhm:\(style.lineHeightMultiple),ls:\(style.lineSpacing),min:\(style.minimumLineHeight),max:\(style.maximumLineHeight))"
+        }
+        if let number = value as? NSNumber {
+            return "num(\(number))"
+        }
+        return String(describing: type(of: value))
+    }
+
+    /// Wraps `content` page-locally and returns the chapter-relative end of
+    /// line `lineIndex`, or nil if it cannot be produced.
+    private static func lineEnd(
+        wrapping content: NSAttributedString,
+        lineIndex: Int,
+        width: CGFloat
+    ) -> Int? {
+        guard content.length > 0,
+              let layouter = DTCoreTextLayouter(attributedString: content) else { return nil }
+        layouter.shouldCacheLayoutFrames = false
+        let frame = layouter.layoutFrame(
+            with: CGRect(x: 0, y: 0, width: width, height: 4_000_000),
+            range: NSRange(location: 0, length: content.length)
+        )
+        guard let lines = frame?.lines as? [DTCoreTextLayoutLine],
+              lineIndex < lines.count else { return nil }
+        return NSMaxRange(lines[lineIndex].stringRange())
+    }
+
+    private static func safeSubstring(_ text: NSString, _ range: NSRange) -> String {
+        guard range.location >= 0, NSMaxRange(range) <= text.length else { return "" }
+        return text.substring(with: range)
+            .replacingOccurrences(of: "\n", with: "⏎")
+    }
+}
+#endif

Sources/RDReaderView/EPUBUI/TextPage/RDEPUBTextPageVerticalJustifier.swift

@@ -0,0 +1,57 @@
+import UIKit
+
+#if canImport(DTCoreText)
+import DTCoreText
+
+/// Redistributes the leftover space at the bottom of a page into the gaps
+/// between lines so the last line sits flush with the content bottom edge
+/// (vertical justification), keeping page character ranges untouched.
+///
+/// Mutating each line's `baselineOrigin` is sufficient: DTCoreText derives
+/// line frames, glyph-run frames and attachment positions from it lazily,
+/// so drawing, selection, highlights and hit-testing all stay consistent.
+enum RDEPUBTextPageVerticalJustifier {
+
+    /// Leftover larger than this many typical line advances is kept as
+    /// whitespace instead of being stretched: it usually comes from a whole
+    /// block (image, table) pushed to the next page, and stretching would
+    /// make the line spacing visibly sparse.
+    static let maxStretchLineAdvanceRatio: CGFloat = 1.5
+
+    static func justify(
+        _ layoutFrame: DTCoreTextLayoutFrame,
+        contentHeight: CGFloat,
+        isChapterLastPage: Bool,
+        pixelScale: CGFloat
+    ) {
+        guard !isChapterLastPage,
+              contentHeight > 0,
+              let lines = layoutFrame.lines as? [DTCoreTextLayoutLine],
+              lines.count >= 2,
+              let firstLine = lines.first,
+              let lastLine = lines.last else {
+            return
+        }
+
+        let leftover = contentHeight - lastLine.frame.maxY
+        guard leftover > 0.5 else { return }
+
+        let gapCount = CGFloat(lines.count - 1)
+        let typicalAdvance = (lastLine.baselineOrigin.y - firstLine.baselineOrigin.y) / gapCount
+        guard typicalAdvance > 0,
+              leftover <= typicalAdvance * maxStretchLineAdvanceRatio else {
+            return
+        }
+
+        let scale = max(pixelScale, 1)
+        for (index, line) in lines.enumerated() where index > 0 {
+            // Round each cumulative shift down to the pixel grid so glyphs
+            // stay sharp and the last line never overshoots the bottom edge.
+            let shift = floor(leftover * CGFloat(index) / gapCount * scale) / scale
+            var origin = line.baselineOrigin
+            origin.y += shift
+            line.baselineOrigin = origin
+        }
+    }
+}
+#endif