fix(reader): stabilize pagination display and page curl reuse

Sources/RDReaderView/EPUBUI/RDEPUBTextContentView.swift

@@ -114,6 +114,7 @@ final class RDEPUBTextContentView: UIView {
 
     private var coreTextDisplayContent: NSAttributedString?
     private var coreTextDisplayRange: NSRange?
+    private var coreTextDisplayOffsetBase: Int = 0
 #endif
 
     private let interactionController = RDEPUBPageInteractionController()
@@ -256,7 +257,7 @@ final class RDEPUBTextContentView: UIView {
         coverImageView.isHidden = true
         coverImageView.image = nil
 
-        let selectionContent = NSMutableAttributedString(attributedString: page.content)
+        let selectionContent = normalizedPageContent(from: page)
         let selectionRange = NSRange(location: 0, length: selectionContent.length)
         selectionContent.addAttribute(
             .foregroundColor,
@@ -265,7 +266,10 @@ final class RDEPUBTextContentView: UIView {
         )
 
 #if canImport(DTCoreText)
-        let displayContent = NSMutableAttributedString(attributedString: page.content)
+        let usesChapterContextDisplay = shouldUseChapterContextDisplay(for: page)
+        let displayContent = usesChapterContextDisplay
+            ? NSMutableAttributedString(attributedString: page.chapterContent)
+            : normalizedPageContent(from: page)
         let fullRange = NSRange(location: 0, length: displayContent.length)
         displayContent.addAttribute(
             .foregroundColor,
@@ -275,7 +279,8 @@ final class RDEPUBTextContentView: UIView {
         coreTextContentView.isHidden = false
         coreTextContentView.backgroundColor = .clear
         coreTextDisplayContent = displayContent
-        coreTextDisplayRange = NSRange(location: 0, length: displayContent.length)
+        coreTextDisplayRange = usesChapterContextDisplay ? page.contentRange : NSRange(location: 0, length: displayContent.length)
+        coreTextDisplayOffsetBase = usesChapterContextDisplay ? 0 : page.pageStartOffset
         textView.isHidden = true
         textView.isUserInteractionEnabled = false
         textView.attributedText = nil
@@ -646,6 +651,47 @@ final class RDEPUBTextContentView: UIView {
         return proxy
     }
 
+    private func normalizedPageContent(from page: RDEPUBTextPage) -> NSMutableAttributedString {
+        let content = NSMutableAttributedString(attributedString: page.content)
+        guard shouldNormalizeContinuationParagraph(for: page) else {
+            return content
+        }
+
+        let text = content.string as NSString
+        let firstParagraphRange = text.paragraphRange(for: NSRange(location: 0, length: 0))
+        guard firstParagraphRange.length > 0 else {
+            return content
+        }
+
+        content.enumerateAttribute(.paragraphStyle, in: firstParagraphRange) { value, range, _ in
+            guard let style = value as? NSParagraphStyle else { return }
+            let mutableStyle = (style.mutableCopy() as? NSMutableParagraphStyle) ?? NSMutableParagraphStyle()
+            mutableStyle.firstLineHeadIndent = mutableStyle.headIndent
+            mutableStyle.paragraphSpacingBefore = 0
+            content.addAttribute(.paragraphStyle, value: mutableStyle.copy() as Any, range: range)
+        }
+
+        return content
+    }
+
+    private func shouldNormalizeContinuationParagraph(for page: RDEPUBTextPage) -> Bool {
+        let pageStart = page.pageStartOffset
+        guard pageStart > 0, pageStart < page.chapterContent.length else {
+            return false
+        }
+
+        let chapterText = page.chapterContent.string as NSString
+        guard let previousScalar = UnicodeScalar(chapterText.character(at: pageStart - 1)) else {
+            return false
+        }
+
+        return !CharacterSet.newlines.contains(previousScalar)
+    }
+
+    private func shouldUseChapterContextDisplay(for page: RDEPUBTextPage) -> Bool {
+        page.metadata.attachmentRanges.isEmpty
+    }
+
 #if canImport(DTCoreText)
     private func updateCoreTextLayoutFrameIfNeeded() {
         guard !coreTextContentView.isHidden,
@@ -666,7 +712,11 @@ final class RDEPUBTextContentView: UIView {
         layouter.shouldCacheLayoutFrames = false
         let layoutFrame = layouter.layoutFrame(with: coreTextContentView.bounds, range: displayRange)
         coreTextContentView.layoutFrame = layoutFrame
-        interactionController.configure(layoutFrame: layoutFrame, page: page)
+        interactionController.configure(
+            layoutFrame: layoutFrame,
+            page: page,
+            contentOffsetBase: coreTextDisplayOffsetBase
+        )
         overlayView.updateSnapshot(interactionController.snapshot)
         backgroundOverlayView.updateSnapshot(interactionController.snapshot)
     }