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InstantPage: underline rendering
Render underline runs in layoutTextItemWithString and position them below the baseline rather than above the text.
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isaac
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2 changed files with 142 additions and 4 deletions
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# InstantPage underline rendering
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## Problem
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`layoutTextItemWithString` in `submodules/InstantPageUI/Sources/InstantPageTextItem.swift` does not handle the `NSAttributedString.Key.underlineStyle` attribute. Underline runs are produced upstream by `InstantPageTextStyleStack.textAttributes()` (`InstantPageTextStyleStack.swift:194-202`) for two distinct sources:
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1. Explicit `RichText.underline` runs (push at `InstantPageTextItem.swift:607`, plus `:628` and `:657` for related cases).
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2. Links whose computed foreground color matches the body-text color — the styleStack falls back to underlining them so they remain distinguishable (`InstantPageTextStyleStack.swift:200-201`).
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The attribute lands on the attributed string in both cases, but the per-line attribute enumerator at `InstantPageTextItem.swift:915-938` only branches on `strikethroughStyle`, `InstantPageMarkerColorAttribute`, and `InstantPageAnchorAttribute`. Underline runs are silently dropped during layout, so they never get drawn.
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The canonical handling pattern lives in `submodules/Display/Source/TextNode.swift:2061-2066` (collection during layout) and `:2619-2638` (manual draw at draw time). `TextNode` deliberately draws underlines manually (`drawUnderlinesManually = true` at `:216`) rather than letting Core Text render them, because CT's underline rendering has historic positioning, color, and clipping issues across glyph clusters and emoji.
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## Goal
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Render underlines in InstantPage articles wherever the styleStack emits `underlineStyle`, matching `TextNode.swift` line-for-line so a future reader sees the same shape in both files.
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## Non-goals
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- Wavy or double underline support — the InstantPage styleStack only emits `NSUnderlineStyle.single`.
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- Changes to `InstantPageTextStyleStack` — the attribute it produces is already correct.
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- Changes to the existing strikethrough draw's reliance on the context's residual fill color — out of scope, not regressed.
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- Changes to `attributesAtPoint` or selection-rect logic — these read attributes directly off `attributedString`, so they already work for underlined ranges.
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## Design
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### New type
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In `InstantPageTextItem.swift`, alongside `InstantPageTextStrikethroughItem`:
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```swift
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struct InstantPageTextUnderlineItem {
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let frame: CGRect
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let range: NSRange
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let color: UIColor?
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}
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```
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`color` carries an optional `NSAttributedString.Key.underlineColor`. There is no `style` field — `.single` is the only value the styleStack emits. `range` is needed at draw time to look up `foregroundColor` per-range when `underlineColor` is absent.
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### Line storage
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Add `let underlineItems: [InstantPageTextUnderlineItem]` to `InstantPageTextLine`, with a matching init parameter alongside `strikethroughItems`. There is exactly one construction site for `InstantPageTextLine` (`InstantPageTextItem.swift:970`), so updating the initializer is local.
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### Collection (layoutTextItemWithString)
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In the `enumerateAttributes` loop currently at `InstantPageTextItem.swift:915-938`, add a parallel branch that mirrors `TextNode.swift:2061-2066`:
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```swift
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if let _ = attributes[NSAttributedString.Key.underlineStyle] {
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let lowerX = floor(CTLineGetOffsetForStringIndex(line, range.location, nil))
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let upperX = ceil(CTLineGetOffsetForStringIndex(line, range.location + range.length, nil))
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let x = lowerX < upperX ? lowerX : upperX
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underlineItems.append(InstantPageTextUnderlineItem(
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frame: CGRect(x: workingLineOrigin.x + x, y: workingLineOrigin.y, width: abs(upperX - lowerX), height: fontLineHeight),
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range: range,
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color: attributes[NSAttributedString.Key.underlineColor] as? UIColor
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))
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}
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```
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Geometry is verbatim the strikethrough branch's — same `lowerX`/`upperX` clamp and the same `workingLineOrigin.x + x` offset. The collection is independent of strikethrough; both branches can fire on the same range.
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### Draw (drawInTile)
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After the strikethrough draw block at `InstantPageTextItem.swift:261-266`, add:
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```swift
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if !line.underlineItems.isEmpty {
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for item in line.underlineItems {
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var color: UIColor? = item.color
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if color == nil {
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self.attributedString.enumerateAttributes(in: item.range, options: []) { attributes, _, _ in
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if let foreground = attributes[NSAttributedString.Key.foregroundColor] as? UIColor {
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color = foreground
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}
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}
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}
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if let color {
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context.setFillColor(color.cgColor)
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}
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let itemFrame = item.frame.offsetBy(dx: lineFrame.minX, dy: 0.0)
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context.fill(CGRect(x: itemFrame.minX, y: itemFrame.minY + 1.0, width: itemFrame.size.width, height: 1.0))
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}
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}
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```
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Color resolution order (`underlineColor` → per-range `foregroundColor`) and position rule (`y: minY + 1.0`, `height: 1.0`) match `TextNode.swift:2624-2638` exactly.
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The `setFillColor` call is gated on a non-nil resolved color so we do not silently flip an unrelated drawing's fill color if no foreground attribute is found in the range. In practice the attributed string always carries a `foregroundColor` (set unconditionally by the styleStack at `InstantPageTextStyleStack.swift:198-211`), so the gate is defense-in-depth, not a hot path.
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## Verification
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- Full Bazel build via `Make.py … --configuration=debug_sim_arm64`.
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- Manual smoke against an article whose body contains an explicit `<u>` block, and a separate article whose link color matches the body color (the styleStack's link-fallback case).
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No unit tests exist in this project (per `CLAUDE.md`).
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## Risk
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Additive: a new struct, a new optional field on `InstantPageTextLine`, one new branch in the layout enumerator, one new draw block. No public API changes, no signature changes outside `InstantPageTextItem.swift`. Runs without `underlineStyle` are unaffected.
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@ -33,6 +33,12 @@ struct InstantPageTextStrikethroughItem {
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let frame: CGRect
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}
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struct InstantPageTextUnderlineItem {
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let frame: CGRect
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let range: NSRange
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let color: UIColor?
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}
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struct InstantPageTextImageItem {
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let frame: CGRect
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let range: NSRange
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@ -68,17 +74,19 @@ public final class InstantPageTextLine {
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let range: NSRange
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public let frame: CGRect
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let strikethroughItems: [InstantPageTextStrikethroughItem]
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let underlineItems: [InstantPageTextUnderlineItem]
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let markedItems: [InstantPageTextMarkedItem]
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let imageItems: [InstantPageTextImageItem]
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let formulaItems: [InstantPageTextFormulaRun]
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public let anchorItems: [InstantPageTextAnchorItem]
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let isRTL: Bool
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init(line: CTLine, range: NSRange, frame: CGRect, strikethroughItems: [InstantPageTextStrikethroughItem], markedItems: [InstantPageTextMarkedItem], imageItems: [InstantPageTextImageItem], formulaItems: [InstantPageTextFormulaRun], anchorItems: [InstantPageTextAnchorItem], isRTL: Bool) {
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init(line: CTLine, range: NSRange, frame: CGRect, strikethroughItems: [InstantPageTextStrikethroughItem], underlineItems: [InstantPageTextUnderlineItem], markedItems: [InstantPageTextMarkedItem], imageItems: [InstantPageTextImageItem], formulaItems: [InstantPageTextFormulaRun], anchorItems: [InstantPageTextAnchorItem], isRTL: Bool) {
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self.line = line
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self.range = range
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self.frame = frame
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self.strikethroughItems = strikethroughItems
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self.underlineItems = underlineItems
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self.markedItems = markedItems
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self.imageItems = imageItems
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self.formulaItems = formulaItems
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@ -264,6 +272,24 @@ public final class InstantPageTextItem: InstantPageItem {
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context.fill(CGRect(x: itemFrame.minX, y: itemFrame.minY + floor((lineFrame.size.height / 2.0) + 1.0), width: itemFrame.size.width, height: 1.0))
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}
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}
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if !line.underlineItems.isEmpty {
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for item in line.underlineItems {
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var color: UIColor? = item.color
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if color == nil {
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self.attributedString.enumerateAttributes(in: item.range, options: []) { attributes, _, _ in
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if let foreground = attributes[NSAttributedString.Key.foregroundColor] as? UIColor {
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color = foreground
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}
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}
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}
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if let color {
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context.setFillColor(color.cgColor)
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}
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let itemFrame = item.frame.offsetBy(dx: lineFrame.minX, dy: 0.0)
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context.fill(CGRect(x: itemFrame.minX, y: itemFrame.minY + lineFrame.size.height + 1.0, width: itemFrame.size.width, height: 1.0))
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}
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}
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}
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context.restoreGState()
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@ -909,9 +935,10 @@ func layoutTextItemWithString(_ string: NSAttributedString, boundingWidth: CGFlo
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}
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var strikethroughItems: [InstantPageTextStrikethroughItem] = []
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var underlineItems: [InstantPageTextUnderlineItem] = []
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var markedItems: [InstantPageTextMarkedItem] = []
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var anchorItems: [InstantPageTextAnchorItem] = []
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string.enumerateAttributes(in: lineRange, options: []) { attributes, range, _ in
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if let _ = attributes[NSAttributedString.Key.strikethroughStyle] {
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let lowerX = floor(CTLineGetOffsetForStringIndex(line, range.location, nil))
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@ -919,6 +946,16 @@ func layoutTextItemWithString(_ string: NSAttributedString, boundingWidth: CGFlo
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let x = lowerX < upperX ? lowerX : upperX
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strikethroughItems.append(InstantPageTextStrikethroughItem(frame: CGRect(x: workingLineOrigin.x + x, y: workingLineOrigin.y, width: abs(upperX - lowerX), height: fontLineHeight)))
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}
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if let _ = attributes[NSAttributedString.Key.underlineStyle] {
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let lowerX = floor(CTLineGetOffsetForStringIndex(line, range.location, nil))
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let upperX = ceil(CTLineGetOffsetForStringIndex(line, range.location + range.length, nil))
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let x = lowerX < upperX ? lowerX : upperX
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underlineItems.append(InstantPageTextUnderlineItem(
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frame: CGRect(x: workingLineOrigin.x + x, y: workingLineOrigin.y, width: abs(upperX - lowerX), height: fontLineHeight),
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range: range,
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color: attributes[NSAttributedString.Key.underlineColor] as? UIColor
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))
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}
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if let color = attributes[NSAttributedString.Key.init(rawValue: InstantPageMarkerColorAttribute)] as? UIColor {
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var lineHeight = fontLineHeight
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var delta: CGFloat = 0.0
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@ -967,7 +1004,7 @@ func layoutTextItemWithString(_ string: NSAttributedString, boundingWidth: CGFlo
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}
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}
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}
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let textLine = InstantPageTextLine(line: line, range: lineRange, frame: CGRect(x: workingLineOrigin.x, y: workingLineOrigin.y, width: lineWidth, height: height), strikethroughItems: strikethroughItems, markedItems: markedItems, imageItems: lineImageItems, formulaItems: lineFormulaItems, anchorItems: anchorItems, isRTL: isRTL)
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let textLine = InstantPageTextLine(line: line, range: lineRange, frame: CGRect(x: workingLineOrigin.x, y: workingLineOrigin.y, width: lineWidth, height: height), strikethroughItems: strikethroughItems, underlineItems: underlineItems, markedItems: markedItems, imageItems: lineImageItems, formulaItems: lineFormulaItems, anchorItems: anchorItems, isRTL: isRTL)
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lines.append(textLine)
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imageItems.append(contentsOf: lineImageItems)
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