fix: only compose matching inner map source in remapping

compose_pair previously returned the inner map for every outer source,
so unrelated sources were incorrectly rewritten through the inner map.
Restrict the match to outer sources that equal inner.file (or the sole
outer source when inner.file is unset), leaving passthrough sources
untouched.

Author: BartWaardenburg

crates/remapping/src/lib.rs

@@ -825,7 +825,23 @@ pub fn remap_chain(maps: &[&SourceMap]) -> Option<SourceMap> {
 /// Compose two source maps: outer maps generated → intermediate, inner maps intermediate → original.
 /// All sources in outer are resolved through inner.
 fn compose_pair(outer: &SourceMap, inner: &SourceMap) -> SourceMap {
-    remap(outer, |_source| Some(inner.clone()))
+    let fallback_source = if inner.file.is_none() {
+        let mut sources = outer.sources.iter().filter(|source| !source.is_empty());
+        match (sources.next(), sources.next()) {
+            (Some(source), None) => Some(source.clone()),
+            _ => None,
+        }
+    } else {
+        None
+    };
+
+    remap(outer, |source| {
+        if inner.file.as_deref() == Some(source) || fallback_source.as_deref() == Some(source) {
+            Some(inner.clone())
+        } else {
+            None
+        }
+    })
 }
 
 /// Per-source entry for streaming variant.
@@ -1936,6 +1952,29 @@ mod tests {
         assert_eq!(loc.line, 1);
     }
 
+    #[test]
+    fn remap_chain_only_composes_matching_inner_file() {
+        let inner = SourceMap::from_json(
+            r#"{"version":3,"file":"intermediate.js","sources":["original.js"],"names":[],"mappings":"AAAA"}"#,
+        )
+        .unwrap();
+        let outer = SourceMap::from_json(
+            r#"{"version":3,"file":"output.js","sources":["intermediate.js","passthrough.js"],"names":[],"mappings":"AAAA,KCAA"}"#,
+        )
+        .unwrap();
+
+        let result = remap_chain(&[&outer, &inner]).unwrap();
+
+        assert!(result.sources.contains(&"original.js".to_string()));
+        assert!(result.sources.contains(&"passthrough.js".to_string()));
+
+        let remapped = result.original_position_for(0, 0).unwrap();
+        assert_eq!(result.source(remapped.source), "original.js");
+
+        let passthrough = result.original_position_for(0, 5).unwrap();
+        assert_eq!(result.source(passthrough.source), "passthrough.js");
+    }
+
     // ── Empty-string source filtering ────────────────────────────
 
     #[test]