fix: Hanging indent (negative text-indent) and em-unit sizing (#1229)

## Summary

* **What is the goal of this PR?** Fixing two independent CSS rendering
bugs combined to make hanging-indent list styles
(e.g. margin-left:3em; text-indent:-1em) render incorrectly:

* **What changes are included?**
1. Negative text-indent was silently ignored

Three guards in ParsedText.cpp (computeLineBreaks,
computeHyphenatedLineBreaks,
extractLine) conditioned firstLineIndent on blockStyle.textIndent > 0,
so any
negative value collapsed to zero. Additionally, wordXpos was uint16_t,
which
cannot represent negative offsets — a cast of e.g. −18 would wrap to
65518 and
render the word far off-screen.

2. extraParagraphSpacing suppressed hanging indents

Even after removing the > 0 guard, the existing !extraParagraphSpacing
condition
would still suppress all text-indent when that setting is on (its
default). Positive
text-indent is a decorative paragraph indent that the user can
reasonably replace with
vertical spacing — negative text-indent is structural (it positions the
list marker)
and must always apply.

3. em unit was calibrated against line height, not font size

emSize was computed as getLineHeight() * lineCompression (the full line
advance).
CSS em units are defined relative to the font-size, which corresponds to
the
ascender height — not the line height. Using line height makes every
em-based
margin/indent ~20–30% wider than a browser would render it, and is
especially
noticeable for CSS that uses font-size: small (which we do not
implement).

## Additional Context

Test case
```
.lsl1 { margin-left: 3em; text-indent: -1em; }

<div class="lsl1">• First list item that wraps across lines</div>
<div class="lsl1">• Short item</div>
```
Before: all lines of all items started at 3 em from the left edge
(indent ignored).

After: the bullet marker hangs at 2 em; continuation lines align at 3
em.

<img width="240" alt="before"
src="https://github.com/user-attachments/assets/9dcbf3e0-fcd9-4af8-b451-a90ba4d2fb75"
/>
<img width="240" alt="after"
src="https://github.com/user-attachments/assets/1ffdcf56-a180-4267-9590-c60d7ac44707"
/>

---

### AI Usage

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Did you use AI tools to help write this code? _**YES**_

authored by

jpirnay and committed by

GitHub 4 months ago aff93f1d f0a549b6

+25 -16

5 changed files

expand all

lib

Epub

ParsedText.cpp

Section.cpp

blocks

TextBlock.cpp

TextBlock.h

parsers

ChapterHtmlSlimParser.cpp

+19 -9

lib/Epub/Epub/ParsedText.cpp

··· 144 144 return {}; 145 145 } 146 146 147 - // Calculate first line indent (only for left/justified text without extra paragraph spacing) 147 + // Calculate first line indent (only for left/justified text). 148 + // Positive text-indent (paragraph indent) is suppressed when extraParagraphSpacing is on. 149 + // Negative text-indent (hanging indent, e.g. margin-left:3em; text-indent:-1em) always applies — 150 + // it is structural (positions the bullet/marker), not decorative. 148 151 const int firstLineIndent = 149 - blockStyle.textIndent > 0 && !extraParagraphSpacing && 152 + blockStyle.textIndentDefined && (blockStyle.textIndent < 0 || !extraParagraphSpacing) && 150 153 (blockStyle.alignment == CssTextAlign::Justify || blockStyle.alignment == CssTextAlign::Left) 151 154 ? blockStyle.textIndent 152 155 : 0; ··· 275 278 const int pageWidth, const int spaceWidth, 276 279 std::vector<uint16_t>& wordWidths, 277 280 std::vector<bool>& continuesVec) { 278 - // Calculate first line indent (only for left/justified text without extra paragraph spacing) 281 + // Calculate first line indent (only for left/justified text). 282 + // Positive text-indent (paragraph indent) is suppressed when extraParagraphSpacing is on. 283 + // Negative text-indent (hanging indent, e.g. margin-left:3em; text-indent:-1em) always applies — 284 + // it is structural (positions the bullet/marker), not decorative. 279 285 const int firstLineIndent = 280 - blockStyle.textIndent > 0 && !extraParagraphSpacing && 286 + blockStyle.textIndentDefined && (blockStyle.textIndent < 0 || !extraParagraphSpacing) && 281 287 (blockStyle.alignment == CssTextAlign::Justify || blockStyle.alignment == CssTextAlign::Left) 282 288 ? blockStyle.textIndent 283 289 : 0; ··· 443 449 const size_t lastBreakAt = breakIndex > 0 ? lineBreakIndices[breakIndex - 1] : 0; 444 450 const size_t lineWordCount = lineBreak - lastBreakAt; 445 451 446 - // Calculate first line indent (only for left/justified text without extra paragraph spacing) 452 + // Calculate first line indent (only for left/justified text). 453 + // Positive text-indent (paragraph indent) is suppressed when extraParagraphSpacing is on. 454 + // Negative text-indent (hanging indent, e.g. margin-left:3em; text-indent:-1em) always applies — 455 + // it is structural (positions the bullet/marker), not decorative. 447 456 const bool isFirstLine = breakIndex == 0; 448 457 const int firstLineIndent = 449 - isFirstLine && blockStyle.textIndent > 0 && !extraParagraphSpacing && 458 + isFirstLine && blockStyle.textIndentDefined && (blockStyle.textIndent < 0 || !extraParagraphSpacing) && 450 459 (blockStyle.alignment == CssTextAlign::Justify || blockStyle.alignment == CssTextAlign::Left) 451 460 ? blockStyle.textIndent 452 461 : 0; ··· 485 494 ? spareSpace / static_cast<int>(actualGapCount) 486 495 : 0; 487 496 488 - // Calculate initial x position (first line starts at indent for left/justified text) 489 - auto xpos = static_cast<uint16_t>(firstLineIndent); 497 + // Calculate initial x position (first line starts at indent for left/justified text; 498 + // may be negative for hanging indents, e.g. margin-left:3em; text-indent:-1em). 499 + auto xpos = static_cast<int16_t>(firstLineIndent); 490 500 if (blockStyle.alignment == CssTextAlign::Right) { 491 501 xpos = effectivePageWidth - lineWordWidthSum - totalNaturalGaps; 492 502 } else if (blockStyle.alignment == CssTextAlign::Center) { ··· 495 505 496 506 // Pre-calculate X positions for words 497 507 // Continuation words attach to the previous word with no space before them 498 - std::vector<uint16_t> lineXPos; 508 + std::vector<int16_t> lineXPos; 499 509 lineXPos.reserve(lineWordCount); 500 510 501 511 for (size_t wordIdx = 0; wordIdx < lineWordCount; wordIdx++) {

+1 -1

lib/Epub/Epub/Section.cpp

··· 10 10 #include "parsers/ChapterHtmlSlimParser.h" 11 11 12 12 namespace { 13 - constexpr uint8_t SECTION_FILE_VERSION = 14; 13 + constexpr uint8_t SECTION_FILE_VERSION = 16; 14 14 constexpr uint32_t HEADER_SIZE = sizeof(uint8_t) + sizeof(int) + sizeof(float) + sizeof(bool) + sizeof(uint8_t) + 15 15 sizeof(uint16_t) + sizeof(uint16_t) + sizeof(uint16_t) + sizeof(bool) + sizeof(bool) + 16 16 sizeof(uint32_t);

+1 -1

lib/Epub/Epub/blocks/TextBlock.cpp

··· 74 74 std::unique_ptr<TextBlock> TextBlock::deserialize(FsFile& file) { 75 75 uint16_t wc; 76 76 std::vector<std::string> words; 77 - std::vector<uint16_t> wordXpos; 77 + std::vector<int16_t> wordXpos; 78 78 std::vector<EpdFontFamily::Style> wordStyles; 79 79 BlockStyle blockStyle; 80 80

+2 -2

lib/Epub/Epub/blocks/TextBlock.h

··· 13 13 class TextBlock final : public Block { 14 14 private: 15 15 std::vector<std::string> words; 16 - std::vector<uint16_t> wordXpos; 16 + std::vector<int16_t> wordXpos; 17 17 std::vector<EpdFontFamily::Style> wordStyles; 18 18 BlockStyle blockStyle; 19 19 20 20 public: 21 - explicit TextBlock(std::vector<std::string> words, std::vector<uint16_t> word_xpos, 21 + explicit TextBlock(std::vector<std::string> words, std::vector<int16_t> word_xpos, 22 22 std::vector<EpdFontFamily::Style> word_styles, const BlockStyle& blockStyle = BlockStyle()) 23 23 : words(std::move(words)), 24 24 wordXpos(std::move(word_xpos)),

+2 -3

lib/Epub/Epub/parsers/ChapterHtmlSlimParser.cpp

··· 278 278 279 279 int displayWidth = 0; 280 280 int displayHeight = 0; 281 - const float emSize = 282 - static_cast<float>(self->renderer.getLineHeight(self->fontId)) * self->lineCompression; 281 + const float emSize = static_cast<float>(self->renderer.getFontAscenderSize(self->fontId)); 283 282 CssStyle imgStyle = self->cssParser ? self->cssParser->resolveStyle("img", classAttr) : CssStyle{}; 284 283 // Merge inline style (e.g. style="height: 2em") so it overrides stylesheet rules 285 284 if (!styleAttr.empty()) { ··· 505 504 } 506 505 } 507 506 508 - const float emSize = static_cast<float>(self->renderer.getLineHeight(self->fontId)) * self->lineCompression; 507 + const float emSize = static_cast<float>(self->renderer.getFontAscenderSize(self->fontId)); 509 508 const auto userAlignmentBlockStyle = BlockStyle::fromCssStyle( 510 509 cssStyle, emSize, static_cast<CssTextAlign>(self->paragraphAlignment), self->viewportWidth); 511 510

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