Missing glyphs trigger fallback and change layout

What it is

Font fallback layout change occurs when a preferred font cannot cover a text segment's characters or shaping needs and the text system forms local runs with another font whose advance widths, face size, baseline, or vertical metrics differ. A failure does not always appear as a .notdef tofu box; visible text may already contain fallback runs. Stable fallback is distinct from an asynchronous web-font swap and its layout shift.

Why it happens

Text engines commonly itemize by script, language, style, and direction, match fonts for runs or shaping clusters, and then shape them; exact staging varies and is not safely modeled as one-character substitution. Complex scripts require joining, reordering, positioning, and combining marks to remain coherent within a cluster. A fallback face's advance widths can change wrapping. Different vertical metrics may enlarge a content-driven line box, while fixed line height may instead clip or overlap glyphs.

Where it stops holding

A font's advertised Unicode range does not prove correct language forms, combining sequences, or OpenType behavior. Matching and fallback vary with script, language metadata, operating-system version, and installed fonts, so success on a developer machine is weak evidence. Variable-font axes, web-font swaps, and synthetic bold can produce similar geometric changes without missing-glyph fallback and should be diagnosed separately.

Applying it

• Specify preferred and fallback fonts per supported script, checking required characters, combining marks, and shaping features rather than font names alone.
• Match fallback face size, baseline, and line metrics where possible; do not size critical components to a tight fit in one font.
• Build coverage strings from target-language copy, names, currency symbols, and rare but valid characters, then render them on every supported platform.
• Inspect .notdef tofu, local fallback runs, and cluster integrity, and monitor web-font swaps separately; require no missing content, malformed wrapping, clipping, or unusable component geometry.

Related

• Same group: S1.03.2 The same character can have different glyph forms across regions · S1.03.3 Emoji and symbol rendering varies across platforms · S1.03.4 Character count is not proportional to display width
• Adjacent: S1.01 Text expansion and layout resilience · S4.03 Multilingual testing and pseudolocalization
• Search terms: font fallback · glyph coverage · text shaping