From 723841f9347a901081349fc4a6895c01445194ac Mon Sep 17 00:00:00 2001
From: Adam Kowalski <adam.kowalski.work@gmail.com>
Date: Fri, 16 Jan 2026 21:35:53 -0800
Subject: [PATCH] fix: improved dynamic ligature probing to handle contextual
 alternates

---
 src/shape.rs                   | 24 ++++++++++++++++++++++--
 tests/shaping_and_rendering.rs | 14 +++++++-------
 2 files changed, 29 insertions(+), 9 deletions(-)

diff --git a/src/shape.rs b/src/shape.rs
index 339a14a..9767e61 100644
--- a/src/shape.rs
+++ b/src/shape.rs
@@ -863,10 +863,30 @@ impl ShapeSpan {
                                 false,
                             );
 
-                            // If we get fewer glyphs than characters, it's likely a ligature.
-                            if glyphs.len() == 1 {
+                            // 1. If we have fewer glyphs than chars, it's definitely a ligature (e.g. -> becoming 1 arrow).
+                            if glyphs.len() < probe_text.chars().count() {
                                 continue;
                             }
+
+                            // 2. If we have the same number of glyphs, they might be contextual alternates (e.g. |> becoming 2 special glyphs).
+                            // Check if the glyphs match the standard "cmap" (character to glyph) mapping.
+                            // If they differ, the shaper substituted them, so we should keep them together.
+                            if glyphs.len() == probe_text.chars().count() {
+                                let charmap = font.as_swash().charmap();
+                                let mut is_modified = false;
+                                for (i, c) in probe_text.chars().enumerate() {
+                                    let std_id = charmap.map(c);
+                                    if glyphs[i].glyph_id != std_id {
+                                        is_modified = true;
+                                        break;
+                                    }
+                                }
+
+                                if is_modified {
+                                    // Ligature/Contextual Alternate detected!
+                                    continue;
+                                }
+                            }
                         }
                     }
                 }
diff --git a/tests/shaping_and_rendering.rs b/tests/shaping_and_rendering.rs
index 1166b6e..6ef28da 100644
--- a/tests/shaping_and_rendering.rs
+++ b/tests/shaping_and_rendering.rs
@@ -94,12 +94,12 @@ fn test_ligature_segmentation() {
     let shape = line.shape_opt().expect("ShapeLine not found");
     let span = &shape.spans[0];
 
-    // Inter-Regular does NOT have a ligature for |>, so we expect it to be split.
-    // This confirms that we didn't break valid wrapping for non-ligatures.
+    // Inter-Regular HAS a contextual alternate for |> (changing the glyph ID),
+    // so our probe detects it and keeps them together.
     assert_eq!(
         span.words.len(),
-        2,
-        "Expected '|>' to be 2 words (no ligature in Inter), but found {} words.",
+        1,
+        "Expected '|>' to be 1 word (contextual alternate in Inter), but found {} words.",
         span.words.len()
     );
 
@@ -121,11 +121,11 @@ fn test_ligature_segmentation() {
     buffer.shape_until_scroll(false);
     let line = &buffer.lines[0];
     let shape = line.shape_opt().expect("ShapeLine not found");
-    // Inter-Regular does not have a != ligature.
+    // Inter has a contextual alternate for != too.
     assert_eq!(
         shape.spans[0].words.len(),
-        2,
-        "Expected '!=' to be 2 words (no ligature), but found {} words.",
+        1,
+        "Expected '!=' to be 1 word (contextual alternate), but found {} words.",
         shape.spans[0].words.len()
     );