gh-152942: Split the source into lines at newlines only

str.splitlines() also breaks at form feed and similar characters that
the parser and the Text widget keep within the line, which misaligned
the column mapping for the following lines.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/idlelib/astbrowser.py

@@ -149,7 +149,8 @@ def populate(self, event=None):
                 first, scope = "1.0", "text"
         self.base = tuple(int(i) for i in first.split("."))
         source = text.get(first, last)
-        self.source_lines = source.splitlines()
+        # Split at "\n" only; str.splitlines() would also break at "\f" etc.
+        self.source_lines = source.split("\n")
         error = count = None
         try:
             tree = ast.parse(source)

Lib/idlelib/idle_test/test_astbrowser.py

@@ -98,6 +98,15 @@ def test_node_range(self):
         item = self.find("Name(id='x'")
         self.assertEqual(self.window.ranges[item], ("4.11", "4.12"))
 
+    def test_form_feed_does_not_shift_lines(self):
+        # gh-152942: a form feed breaks str.splitlines() but not the parser,
+        # so it must not shift source_lines and misplace later nodes.
+        self.text.delete("1.0", "end")
+        self.text.insert("1.0", "x = 1\f\ny = 22\n")   # form feed ends line 1.
+        self.window.populate()
+        item = self.find("Constant(value=22)")
+        self.assertEqual(self.window.ranges[item], ("2.4", "2.6"))
+
     def test_selection_scope(self):
         self.text.tag_add("sel", "4.11", "4.16")   # 'x + 1' on line 4.
         self.window.populate()