common/parser: fix nasty bug causing subtle corruption of generation prompt (#20825)

common/chat-auto-parser-helpers.cpp

@@ -188,6 +188,21 @@ diff_split calculate_diff_split(const std::string & left, const std::string & ri
         result.suffix = "";
         // pick prefix = all as representation
     }
+
+    // When left has no unique content (result.left is empty), left is entirely
+    // shared with right. The simultaneous prefix/suffix segment matching can
+    // incorrectly consume trailing segments of left as suffix when those same
+    // segments also appear at the end of right (e.g. "\n" at the end of both
+    // the shared content and the generation prompt). This rotates the diff.
+    // Fix: if left is a prefix of right, enforce that directly.
+    if (result.left.empty() && !result.right.empty() &&
+            left.size() <= right.size() &&
+            right.substr(0, left.size()) == left) {
+        result.prefix = left;
+        result.suffix = "";
+        result.right  = right.substr(left.size());
+    }
+
     return result;
 }