Fix corner case when clamping lines of modify DiffChunks

src/views/conflicts-resolution/diff.ts

@@ -11,8 +11,8 @@ function diff(oldText: string, newText: string): DiffChunk[] {
   const newLines = newText.split("\n");
   const result: DiffChunk[] = [];
 
-  let i = 0,
-    j = 0;
+  let i = 0;
+  let j = 0;
 
   while (i < oldLines.length || j < newLines.length) {
     // Skip identical lines
@@ -88,16 +88,42 @@ function diff(oldText: string, newText: string): DiffChunk[] {
   }
 
   return mergeDiffs(
-    result.map((chunk) => ({
-      ...chunk,
-      startLeftLine: clampLine(chunk.startLeftLine, oldLines.length),
-      endLeftLine: clampLine(chunk.endLeftLine, oldLines.length),
-      startRightLine: clampLine(chunk.startRightLine, newLines.length),
-      endRightLine: clampLine(chunk.endRightLine, newLines.length),
-    })),
+    result.map((chunk) =>
+      clampChunkLines(chunk, oldLines.length, newLines.length),
+    ),
   );
 }
 
+// Clamp lines in the change and change its type accordingly
+function clampChunkLines(
+  chunk: DiffChunk,
+  oldLinesLength: number,
+  newLinesLength: number,
+): DiffChunk {
+  const startLeftLine = clampLine(chunk.startLeftLine, oldLinesLength);
+  const endLeftLine = clampLine(chunk.endLeftLine, oldLinesLength);
+  const startRightLine = clampLine(chunk.startRightLine, newLinesLength);
+  const endRightLine = clampLine(chunk.endRightLine, newLinesLength);
+  let type = chunk.type;
+  if (type === "modify") {
+    // If we're here it means that the type was modify,
+    // if the lines on one side are the same number
+    // we nee to change the type or the change won't be shown correctly.
+    if (startLeftLine === endLeftLine) {
+      type = "add";
+    } else if (startRightLine === endRightLine) {
+      type = "remove";
+    }
+  }
+  return {
+    type,
+    startLeftLine,
+    endLeftLine,
+    startRightLine,
+    endRightLine,
+  };
+}
+
 // Use to make sure the line number in the diff chunk doesn't exceed
 // the number of lines of the document.
 function clampLine(line: number, maxLines: number): number {