gh-152966: Split the source into lines at newlines only

str.splitlines() also breaks at form feed and similar characters that
the compiler 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/disbrowser.py

@@ -218,7 +218,8 @@ 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 = None
         try:
             code = compile(source, "<disassembly>", "exec")
@@ -346,7 +347,7 @@ def show_running(self, code, offset):
         # Positions map to the editor only when it shows this frame's file.
         self.map_source = self.editor_shows(code.co_filename)
         if self.map_source:
-            self.source_lines = self.text.get("1.0", "end-1c").splitlines()
+            self.source_lines = self.text.get("1.0", "end-1c").split("\n")
         self.add_code(code)   # The frame's own code is the first top-level row.
         self.mark_current(offset)
         self.status.configure(

Lib/idlelib/idle_test/test_disbrowser.py

@@ -123,6 +123,16 @@ def test_instruction_range(self):
         item = self.instr_at(("4.11", "4.12"))
         self.assertEqual(self.window.tree.set(item, "arg"), "x")
 
+    def test_form_feed_does_not_shift_lines(self):
+        # gh-152966: a form feed breaks str.splitlines() but not the compiler,
+        # so it must not shift source_lines and misplace later instructions.
+        window = self.window
+        self.text.delete("1.0", "end")
+        self.text.insert("1.0", "a = 1\f\nb = 22\n")   # form feed ends line 1.
+        window.populate()
+        ranges = [window.ranges[it] for it in window.instr_items]
+        self.assertIn(("2.4", "2.6"), ranges)          # the load of 22.
+
     def code_node(self, qualname):
         "Return the top-level code-object row named qualname."
         tree = self.window.tree