Keep the empty lines in the text fields

It fixes #18036.

src/core/annotation.js

@@ -1292,7 +1292,22 @@ class Annotation {
         bbox,
         matrix
       );
-      this.data.textContent = text;
+
+      if (this instanceof TextWidgetAnnotation) {
+        // The text extractor doesn't handle empty lines correctly, so if the
+        // content we get is more or less (modulo whitespaces) the same as the
+        // field value we just ignore it.
+        const regex = text
+          .join("\n")
+          .replaceAll(/([.*+?^${}()|[\]\\])|(\s+)/g, (_m, p1) =>
+            p1 ? `\\${p1}` : "\\s+"
+          );
+        if (!new RegExp(regex).test(this.data.fieldValue)) {
+          this.data.textContent = text;
+        }
+      } else {
+        this.data.textContent = text;
+      }
     }
   }
 

test/pdfs/.gitignore

@@ -674,3 +674,4 @@
 !issue18072.pdf
 !stamps.pdf
 !issue15096.pdf
+!issue18036.pdf

test/test_manifest.json

@@ -10684,5 +10684,13 @@
         "value": false
       }
     }
+  },
+  {
+    "id": "issue18036",
+    "file": "pdfs/issue18036.pdf",
+    "md5": "940ad97fc1a6f8d288a213af80313c7e",
+    "rounds": 1,
+    "type": "eq",
+    "annotations": true
   }
 ]