fix(features/completion): complete array gaps against the insertion index

A cursor between elements that touched neither fell through to the
element count, so positional schemas (prefixItems, tuple items) matched
the wrong index or none at all.

src/features/completion.rs

@@ -154,17 +154,19 @@ fn descend_array(
     offset: usize,
     path: Vec<Segment>,
 ) -> Option<Context> {
-    let mut count = 0;
+    let mut insert_index = 0;
     for (index, element) in array.elements().enumerate() {
         if element.range.touches(offset) {
             let mut next = path;
             next.push(Segment::Index(index));
             return descend(element, offset, next);
         }
-        count = index + 1;
+        if element.range.start < offset {
+            insert_index = index + 1;
+        }
     }
     let mut next = path;
-    next.push(Segment::Index(count));
+    next.push(Segment::Index(insert_index));
     Some(Context::Value {
         path: next,
         replace: None,
@@ -488,6 +490,16 @@ mod tests {
         assert_eq!(got, vec!["deep"]);
     }
 
+    #[test]
+    fn array_gap_completes_against_its_insertion_index() {
+        let schema = json!({"properties": {"pair": {
+            "prefixItems": [{"type": "boolean"}, {"const": "z"}]
+        }}});
+        // Cursor in the gap between the comma and the second element.
+        let got = labels(r#"{"pair": [true, "z"]}"#, 0, 15, schema);
+        assert_eq!(got, vec!["\"z\""]);
+    }
+
     #[test]
     fn no_proposals_without_matching_schema() {
         let schema = json!({"type": "object"});