Adjust attach() per TeXbook Appendix G 18a (#1129)

library/src/math/attach.rs

@@ -299,6 +299,7 @@ fn compute_shifts_up_and_down(
 
     let mut shift_up = Abs::zero();
     let mut shift_down = Abs::zero();
+    let is_char_box = is_character_box(base);
 
     for e in [tl, tr].into_iter().flatten() {
         let ascent = match &base {
@@ -308,13 +309,13 @@ fn compute_shifts_up_and_down(
 
         shift_up = shift_up
             .max(sup_shift_up)
-            .max(ascent - sup_drop_max)
+            .max(if is_char_box { Abs::zero() } else { ascent - sup_drop_max })
             .max(sup_bottom_min + e.descent());
     }
     for e in [bl, br].into_iter().flatten() {
         shift_down = shift_down
             .max(sub_shift_down)
-            .max(base.descent() + sub_drop_min)
+            .max(if is_char_box { Abs::zero() } else { base.descent() + sub_drop_min })
             .max(e.ascent() - sub_top_max);
     }
 
@@ -339,6 +340,27 @@ fn compute_shifts_up_and_down(
     (shift_up, shift_down)
 }
 
+/// Whether the fragment consists of a single character or atomic piece of text.
+fn is_character_box(fragment: &MathFragment) -> bool {
+    match fragment {
+        MathFragment::Glyph(_) | MathFragment::Variant(_) => {
+            fragment.class() != Some(MathClass::Large)
+        }
+        MathFragment::Frame(fragment) => is_atomic_text_frame(&fragment.frame),
+        _ => false,
+    }
+}
+
+/// Handles e.g. "sin", "log", "exp", "CustomOperator".
+fn is_atomic_text_frame(frame: &Frame) -> bool {
+    // Meta information isn't visible or renderable, so we exclude it.
+    let mut iter = frame
+        .items()
+        .map(|(_, item)| item)
+        .filter(|item| !matches!(item, FrameItem::Meta(_, _)));
+    matches!(iter.next(), Some(FrameItem::Text(_))) && iter.next().is_none()
+}
+
 /// Unicode codepoints that should have sub- and superscripts attached as limits.
 #[rustfmt::skip]
 const LIMITS: &[char] = &[

tests/typ/math/attach.typ

@@ -68,6 +68,16 @@ $ sqrt(a_(1/2)^zeta), sqrt(a_alpha^(1/2)), sqrt(a_(1/2)^(3/4)) \
 // Test frame base.
 $ (-1)^n + (1/2 + 3)^(-1/2) $
 
+---
+#set text(size: 8pt)
+
+// Test that the attachments are aligned horizontally.
+$ x_1 p_1 frak(p)_1 2_1 dot_1 lg_1 !_1 \\_1 ]_1 "ip"_1 op("iq")_1 \
+  x^1 b^1 frak(b)^1 2^1 dot^1 lg^1 !^1 \\^1 ]^1 "ib"^1 op("id")^1 \
+  x_1 y_1 "_"_1 x^1 l^1 "`"^1 attach(I,tl:1,bl:1,tr:1,br:1)
+  scripts(sum)_1^1 integral_1^1 |1/2|_1^1 \
+  x^1_1, "("b y")"^1_1 != (b y)^1_1, "[∫]"_1 [integral]_1 $
+
 ---
 // Test limit.
 $ lim_(n->oo \ n "grows") sum_(k=0 \ k in NN)^n k $