Text runs can be measured narrower than their shaped line width, forcing mid-word breaks in flex rows #181

Summary

A flex-row item holding a <Text> whose content is a nested inline <Text> run can be laid out narrower than the line its breaker actually produces. When the shaped width exceeds the measured box by even a fraction of a point, the breaker falls back to character-level breaks — e.g. Mumbai renders as Mumba / i, and Mumbai, renders as Mumbai / ,.

Root cause

Two measurement paths disagree:

• Intrinsic sizing: measure_intrinsic_width (engine/src/layout/mod.rs) measures text via FontContext::measure_string — a per-character hmtx-width sum plus a 0.01 epsilon.
• Line breaking for styled runs: break_runs_into_lines → measure_styled_chars (engine/src/text/mod.rs) resolves the concrete family per character and shapes contiguous runs with rustybuzz, then sums the shaped cluster widths.

For Mumbai in IBM Plex Serif 10pt the shaped width is ~39.13pt while measure_string reports ~39.03pt (+0.01 epsilon → ~39.04pt box). The styled breaker sees the final i exceed max_width and character-breaks it.

Plain string content does not hit this: break_into_lines → measure_chars takes the same per-character path measure_string uses whenever the font stack lists more than one family, so box and breaker agree. The discrepancy only surfaces for styled runs — any <Text> containing nested <Text> children — because runs always go through measure_styled_chars' shaping path even when the family list resolves to a single concrete font.

Minimal repro

import { Document, Page, View, Text, Font, renderToObject } from "@formepdf/react";

Font.register({ family: "IBM Plex Serif", src: /* IBM Plex Serif regular TTF/OTF bytes or URL */ });

renderToObject(
  <Document>
    <Page size="A4">
      <View style={{ flexDirection: "row", justifyContent: "space-between", width: 200 }}>
        <Text style={{ fontFamily: "IBM Plex Serif", fontSize: 10 }}>Acme</Text>
        <Text style={{ fontFamily: "IBM Plex Serif", fontSize: 10 }}>
          <Text>Mumbai</Text>
        </Text>
      </View>
    </Page>
  </Document>,
);

The trailing text lays out as Mumba / i. Emitting the string directly (no nested <Text>) keeps it on one line. Which strings split depends on the font — Bengaluru also splits under IBM Plex Serif, while Noto Serif is self-consistent for these strings.

Suggested fix direction

Measure run-structured text with the same measure_styled_chars shaping path the breaker uses (or clamp the breaker's available width to the same measure), so a content-measured box can never end up narrower than the shaped line it must hold. Short of that, the breaker's character-level fallback could refuse to split a word when no break opportunity exists within the line.

Found while investigating reactive-resume/reactive-resume#3586 — happy to provide the laid-out ElementInfo dumps (w=39.04 box vs the two forced lines) if useful.