Line Breaking

The line-breaking engine is the heart of Mobile-Pretext. It walks cached segment widths with pure arithmetic to determine where lines break.

Rules

The engine matches CSS white-space: normal + overflow-wrap: break-word:

1. Break before overflow — when the next segment would exceed maxWidth, break before it
2. Trailing whitespace hangs — spaces at the end of a line don't count toward the line width
3. Grapheme-level breaking — words wider than maxWidth break at grapheme cluster boundaries (not byte or codepoint boundaries)

Two Code Paths

Simple fast path

Used when text has no soft hyphens, tabs, preserved spaces, or hard breaks — just regular text and spaces. This covers the vast majority of app text.

Full path

Handles all 8 segment break kinds, including:

• Chunks for hard-break boundaries (pre-wrap mode)
• Soft hyphen logic — tries to fit more text before breaking at a soft hyphen; shows - when broken
• Tab advances — positions to next tab stop
• Preserved spaces — visible width that still allows breaking

Soft Hyphen Behavior

Soft hyphens (\u00AD) are invisible discretionary break points:

"Supercalifragilisticexpialidocious"       → no hyphens shown
"Sup\u{AD}er\u{AD}cal\u{AD}i\u{AD}frag…"  → "Sup-" at break, invisible otherwise

When the engine chooses to break at a soft hyphen:

• The broken line gets a trailing - character
• The line width includes the hyphen width
• The next line starts after the soft hyphen

Grapheme-Level Breaking

When a word is wider than maxWidth (common with CJK text or very narrow containers), the engine breaks at grapheme cluster boundaries:

"Pneumonoultramicroscopicsilicovolcanoconiosis" at 50px width:
  Line 1: "Pneumon"
  Line 2: "oultram"
  Line 3: "icrosco"
  ...

Grapheme clusters are the correct breaking unit because they represent what users perceive as a single character:

• "é" (e + combining accent) → one grapheme, won't break apart
• "👨‍👩‍👧" (family emoji) → one grapheme, won't break apart
• "가" (Korean syllable) → one grapheme

Performance

The line-breaking engine operates on flat arrays with no allocations:

Operation	Cost
layout() (count only)	~0.0002ms per text
layoutWithLines() (with text)	~0.01ms per text
walkLineRanges() (no text)	~0.005ms per text
layoutNextLine() (per line)	~0.001ms per line