Fix crash caused by glyph splitting

crates/resvg/tests/integration/render.rs

@@ -1502,6 +1502,7 @@ use crate::render;
 #[test] fn text_text_filter_bbox() { assert_eq!(render("tests/text/text/filter-bbox"), 0); }
 #[test] fn text_text_ligatures_handling_in_mixed_fonts_1() { assert_eq!(render("tests/text/text/ligatures-handling-in-mixed-fonts-1"), 0); }
 #[test] fn text_text_ligatures_handling_in_mixed_fonts_2() { assert_eq!(render("tests/text/text/ligatures-handling-in-mixed-fonts-2"), 0); }
+#[test] fn text_text_glyph_splitting() { assert_eq!(render("tests/text/text/glyph-splitting"), 0); }
 #[test] fn text_text_mm_coordinates() { assert_eq!(render("tests/text/text/mm-coordinates"), 0); }
 #[test] fn text_text_nested() { assert_eq!(render("tests/text/text/nested"), 0); }
 #[test] fn text_text_no_coordinates() { assert_eq!(render("tests/text/text/no-coordinates"), 0); }

crates/usvg/src/text/layout.rs

@@ -1,7 +1,7 @@
 // Copyright 2022 the Resvg Authors
 // SPDX-License-Identifier: Apache-2.0 OR MIT
 
-use std::collections::HashMap;
+use std::collections::{HashMap, HashSet};
 use std::num::NonZeroU16;
 use std::sync::Arc;
 
@@ -882,6 +882,11 @@ fn process_chunk(
     // but some can use `fi` (U+FB01) instead.
     // Meaning that during merging we have to overwrite not individual glyphs, but clusters.
 
+    // Glyph splitting assigns distinct glyphs to the same index in the original text, we need to
+    // store previously used indices to make sure we do not re-use the same index while overwriting
+    // span glyphs.
+    let mut positions = HashSet::new();
+
     let mut glyphs = Vec::new();
     for span in &chunk.spans {
         let font = match fonts_cache.get(&span.font) {
@@ -904,17 +909,25 @@ fn process_chunk(
             continue;
         }
 
+        positions.clear();
+
         // Overwrite span's glyphs.
         let mut iter = tmp_glyphs.into_iter();
         while let Some(new_glyph) = iter.next() {
             if !span_contains(span, new_glyph.byte_idx) {
                 continue;
             }
 
-            let Some(idx) = glyphs.iter().position(|g| g.byte_idx == new_glyph.byte_idx) else {
+            let Some(idx) = glyphs
+                .iter()
+                .position(|g| g.byte_idx == new_glyph.byte_idx)
+                .filter(|pos| !positions.contains(pos))
+            else {
                 continue;
             };
 
+            positions.insert(idx);
+
             let prev_cluster_len = glyphs[idx].cluster_len;
             if prev_cluster_len < new_glyph.cluster_len {
                 // If the new font represents the same cluster with fewer glyphs