Box family — phantom, smash, lap (items 1-3)

[kostub]

Goal

End-to-end support for the box family: \phantom, \hphantom, \vphantom, \mathstrut, \smash (+[t]/[b]), and \llap/\rlap/\clap (+ \math*lap aliases). Adds a single parameterized atom (MTMathBox, 5-flag matrix) + one display (MTMathBoxDisplay) backing the entire family.

This is PR 1 of the box-and-spacing layer. It is independent of PR 2 and based directly on master.

Commits

• [item 1] Add MTMathBox atom, MTBoxHAlign, and kMTMathAtomBox plumbing
• [item 2] Parse phantom/smash/lap commands into MTMathBox
• [item 3] Render MTMathBox via MTMathBoxDisplay and typesetter box case

Implementation

• Atom model: kMTMathAtomBox = 20 (script-capable), MTBoxHAlign enum, MTMathBox : MTMathAtom with innerList + keepWidth/keepHeight/keepDepth/drawChild/hAlign flags; lossy stringValue/appendLaTeXToString: per the flag matrix.
• Parser: +boxCommands table + dispatch branch in -atomForCommand: (synthetic ( for \mathstrut, optional [t]/[b] for \smash).
• Render: MTMathBoxDisplay : MTDisplay reporting flag-selected geometry and drawing/suppressing/offsetting its child; new kMTMathAtomBox case in MTTypesetter -createDisplayAtoms: (reclassifies to Ordinary, scripts supported).

Tests

• MTMathListTest: 10/10. MTMathListBuilderTest: 146/146. MTTypesetterTest: 108/108.
• Full suite: swift test 354/354, Xcode 313/313. Includes integration (vphantom drives delimiter size, scripts on box, rlap advance, negative-origin lap).

Deviations (documented by implementer)

1. Added a public initWithType:value: declaration on MTMathBox so the guard test can compile (base designated initializer lives in a private category).
2. Added an appendLaTeXToString: override calling self.stringValue (the base does not route through stringValue), mirroring MTMathStyle.

References

• Plan: docs/plans/2026-06-28-box-spacing-layer.md (PR 1, Tasks 1–3)
• LLD: docs/lld/2026-06-28-box-spacing-layer.md

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request introduces support for the box family of LaTeX commands (such as phantom, hphantom, vphantom, mathstrut, smash, and various lap commands) by implementing a new MTMathBox atom type and its corresponding MTMathBoxDisplay rendering class. Feedback on the implementation suggests improving the robustness of the parser when handling optional arguments for smash (e.g., [t] or [b]) by raising a parsing error if the closing bracket is missing.

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[gemini-code-assist]

When parsing the optional argument [t] or [b] for \smash, if the closing bracket ] is missing (e.g., \smash[t at the end of the input), the parser currently continues silently without raising an error. To be consistent with other optional argument parsing (like \sqrt) and standard LaTeX syntax, we should verify that the closing bracket was successfully found and raise a MTParseErrorCharacterNotFound error if it is missing.

            if (ch == '[') {
                NSMutableString* opt = [NSMutableString string];
                BOOL foundClose = NO;
                while ([self hasCharacters]) {
                    unichar c = [self getNextCharacter];
                    if (c == ']') {
                        foundClose = YES;
                        break;
                    }
                    [opt appendString:[NSString stringWithCharacters:&c length:1]];
                }
                if (!foundClose) {
                    [self setError:MTParseErrorCharacterNotFound message:@"Expected character not found: ]"];
                    return nil;
                }
                NSString* o = [opt stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]];
                if ([o isEqualToString:@"t"]) { box.keepHeight = NO; box.keepDepth = YES; }
                else if ([o isEqualToString:@"b"]) { box.keepHeight = YES; box.keepDepth = NO; }
                // any other value: ignore, leave smash-both flags (no crash).

[kostub]

Fixed in 0a8395f. The smash optional-arg loop now tracks the closing bracket and raises MTParseErrorCharacterNotFound when it is missing, matching the \sqrt[…] precedent. Added \smash[t to the parse-error test table.

[kostub]

Code review (orchestrator inline — automated review agent stalled twice on a stream watchdog, so this is a focused static review)

Verdict: solid, no blocking issues. Faithful to the LLD; reuses the colorbox/overline idioms correctly. Full suite already green per the implementer (Xcode 313/313, swift test 354/354). Did not approve or merge.

What's right

• Atom placement kMTMathAtomBox = 20 (< kMTMathAtomBoundary) → script-capable, matching the LLD intent; testScriptOnBox exercises it.
• Typesetter case mirrors overline/accent: flush _currentLine, addInterElementSpace:…currentType:kMTMathAtomOrdinary, then atom.type = kMTMathAtomOrdinary before any spacing lookup — so it never hits the getInterElementSpace default: assert. Pen advances by display.width (0 for vphantom/laps). break (not continue) lets the normal prevNode bookkeeping run with the reclassified type. Correct.
• Display geometry keep* flags map straight to writable width/ascent/descent; phantom suppression is a clean early return after [super draw:]. Lap offsets (-w, -w/2, 0) match the hAlign matrix.
• copyWithZone:/finalized deep-copy innerList and every flag; super preserves the subclass. Good.
• Deviations are justified: the public initWithType:value: declaration is needed because the base designated initializer lives in a private category (else the guard test can't compile); the appendLaTeXToString: override is needed because the base does not route through stringValue — mirrors MTMathStyle.

Non-blocking nits

1. draw: mutates child.position = CGPointZero before drawing the translated child. Harmless (the box exclusively owns its child) but it's a side-effect in draw: that the sibling displays avoid — they keep subdisplay positions immutable and rely on CTM translation alone. Consider translating by position + offset and leaving child.position untouched, or setting it once at init.
2. \smash[ with no closing ] consumes the rest of the input into the optional-arg buffer, then buildInternal: sees EOF and wraps an empty inner. No crash (fine), but silently swallows trailing content. A bounded peek or a ]-not-found fallback (un-look the buffer) would be friendlier. Edge-case only.
3. \mathstrut synthesizes ( via atomForCharacter:'(' (an Ordinary atom) rather than an open-delimiter atom. Height is correct at normal size, so this matches \vphantom{(} in practice; just noting it's a plain glyph, not a delimiter.

None of these block merge.