bug:ENG-20249 fix counter index in getLineInformation

src/compute-lines.ts

@@ -269,9 +269,14 @@ const computeLineInformation = (
         right.value = line;
       }
 
-      counter += 1;
-
+      // `counter` tracks the index of the line inside `lineInformation`, which
+      // is what `diffLines` records. The lookahead pass (`isRetrieveNext`) only
+      // borrows the right side of an already-counted modification line, so it
+      // must not advance the counter - otherwise `diffLines` drifts ahead of
+      // the real line indexes by one for every modification, which pushes the
+      // "Expand x lines ..." indicator below the diff block it belongs to.
       if (!isRetrieveNext) {
+        counter += 1;
         lineInformation.push({
           left,
           right,

test/compute-lines.test.ts

@@ -260,7 +260,7 @@ Also this info`;
           left: {},
         },
       ],
-      diffLines: [0, 2],
+      diffLines: [0, 1],
     });
   });
 
@@ -304,7 +304,7 @@ Also this info`;
           left: {},
         },
       ],
-      diffLines: [0, 2],
+      diffLines: [0, 1],
     });
   });
 
@@ -336,7 +336,7 @@ Also this info`;
           left: {},
         },
       ],
-      diffLines: [0, 2],
+      diffLines: [0, 1],
     });
   });
 
@@ -370,7 +370,7 @@ Also this info`;
           left: {},
         },
       ],
-      diffLines: [0, 2],
+      diffLines: [0, 1],
     });
   });
 });