Two rendering bugs: digit before \; , and digit inside a braced subscript _{...}

[HeinzMattes]

Environment

• LaTeXSwiftUI: main branch, commit cc677f66b9cc1751e09207621d62d1cda0200675 (just after the v2.0.0 tag)
• MathJaxSwift: 3.5.0 (00e9c3df6b1c82031c7fe3785028f3d21c0d73ba)
• SwiftDraw: 0.29.0 (82699f4bdf2f075793cfa0d392eb27c59b41a3fe)
• swift-html-entities: 4.0.1
• Xcode / iOS Simulator, Swift 6

Summary

Two distinct, minimal, reliably reproducible cases where a LaTeX(text) view falls back to
displaying the raw source (.errorMode(.original)) instead of rendering the formula, even though
the input is valid TeX/LaTeX.

Important diagnostic detail: in both cases, calling MathJaxSwift's tex2svg(...) directly on
the exact same (stripped) content succeeds and produces a valid, non-empty SVG, and feeding that
SVG into SwiftDraw.SVG(data:) directly also succeeds. The failure only appears when going through
the full LaTeXSwiftUI LaTeX(text) rendering path (tested both in a real app and in a minimal
one-view isolated test target), so the bug does not appear to be reachable from MathJaxSwift or
SwiftDraw in isolation -- it seems to be specific to how LaTeXSwiftUI drives them.

---

Bug 1: a digit immediately followed by \; (no separator) is not recognized as a spacing command

\; is a TeX control symbol (backslash + exactly one non-letter character), and per the TeX
grammar it should be recognized regardless of what precedes it -- unlike control words (e.g.
\alpha), which need a following space to terminate but are not affected by a preceding
character either. So 3\;x is valid, unambiguous LaTeX.

Minimal repro:

struct ContentView: View {
    // FAILS: shows literal "3\;" as text instead of the expected thin space
    let broken = "\\[ a(3\\; x) \\]"

    // WORKS: adding a space before \; fixes it
    let fixed = "\\[ a(3 \\; x) \\]"

    var body: some View {
        List {
            LaTeX(broken)
            LaTeX(fixed)
        }
    }
}

Observed:

• a(3\; x) -> the standalone \; characters are shown as literal text inline with the rendered formula.
• a(3 \; x) -> renders correctly, \; produces the expected small space.

Workaround we're using: insert a space between a digit and a following \; in a preprocessing
pass before handing text to LaTeX(...).

---

Bug 2: a braced subscript _{...} that contains a digit fails to render at all

Minimal repro:

struct ContentView: View {
    let case1 = "\\[ { E_{1} } \\]"        // FAILS
    let case2 = "\\[ { E_1 } \\]"          // OK (bare, no braces)
    let case3 = "\\[ { E_{Q1} } \\]"       // FAILS
    let case4 = "\\[ { E_{01} } \\]"       // FAILS
    let case5 = "\\[ { E_{p} } \\]"        // OK (braced, but letters only)
    let case6 = "\\[ { E_{\\mathrm{Q1}} } \\]"  // OK (workaround)
    let case7 = "\\[ { E_{\\text{Q1}} } \\]"    // OK (workaround)
    let case8 = "\\[ { E_{Q{1}} } \\]"     // still FAILS (nesting braces around just the digit does not help)

    var body: some View {
        List {
            LaTeX(case1); LaTeX(case2); LaTeX(case3); LaTeX(case4)
            LaTeX(case5); LaTeX(case6); LaTeX(case7); LaTeX(case8)
        }
    }
}

Observed pattern:

• E_1 (bare, no braces) -> renders correctly
• E_{p} (braced, letters only) -> renders correctly
• E_{1} (braced, digit) -> FAILS, raw source shown
• E_{Q1} (braced, letter+digit) -> FAILS
• E_{01} (braced, digits) -> FAILS
• E_{Q{1}} (digit nested in an inner brace) -> FAILS
• E_{\mathrm{Q1}} -> renders correctly
• E_{\text{Q1}} -> renders correctly

So: a braced subscript group fails to render as soon as it contains a digit anywhere in it --
unless that digit is inside a \mathrm{...} or \text{...} wrapper, in which case it renders
correctly (and arguably more correctly typeset, upright rather than italic, for label-like
subscripts).

This is a real-world, frequently-occurring case in scientific/engineering notation -- subscripts
like _{01}, _{Q1}, _{Q2} (naming points, charges, etc.) are extremely common, and this bug
makes every single one of them fail to render.

Workaround we're using: in a preprocessing pass, detect every _{...} group whose content
contains at least one digit and wrap the content in \mathrm{...}, e.g. _{Q1} -> _{\mathrm{Q1}}.
Groups that are purely alphabetic (e.g. _{p}) are left untouched since they already render
correctly.

---

Notes

• Both bugs were found via real content in a German electrical-engineering textbook app (many
  formulas use vector notation with \; for spacing, and multi-character subscripts like _{01}
  to label points/charges), so they are not obscure/contrived edge cases -- they affect very
  ordinary, common LaTeX usage.
• Happy to provide more detail, a fuller reproduction project, or test a patch if useful.