[3.14] gh-159617: Fix _pyrepl dropping the end of long wrapped lines

Lib/_pyrepl/reader.py

@@ -353,14 +353,16 @@ def calc_screen(self) -> list[str]:
             else:
                 pre = prompt
                 prelen = prompt_len
-                for wrap in range(wrapcount + 1):
+                while True:
                     index_to_wrap_before = 0
                     column = 0
                     for char_width in char_widths:
                         if column + char_width + prelen >= self.console.width:
                             break
                         index_to_wrap_before += 1
                         column += char_width
+                    if index_to_wrap_before == 0 and not prelen:
+                        index_to_wrap_before = 1  # force progress
                     if len(chars) > index_to_wrap_before:
                         offset += index_to_wrap_before
                         post = "\\"
@@ -378,6 +380,8 @@ def calc_screen(self) -> list[str]:
                     char_widths = char_widths[index_to_wrap_before:]
                     pre = ""
                     prelen = 0
+                    if not post:
+                        break
         self.screeninfo = screeninfo
         self.cxy = self.pos2xy()
         if self.msg:

Lib/test/test_pyrepl/test_reader.py

@@ -35,6 +35,11 @@ def test_calc_screen_wrap_three_lines(self):
         reader, _ = handle_events_narrow_console(events)
         self.assert_screen_equal(reader, f"{9*"a"}\\\n{9*"a"}\\\naa")
 
+    def test_calc_screen_wrap_three_lines_last_char(self):
+        events = code_to_events(19 * "a")
+        reader, _ = handle_events_narrow_console(events)
+        self.assert_screen_equal(reader, f"{9*"a"}\\\n{9*"a"}\\\na")
+
     def test_calc_screen_prompt_handling(self):
         def prepare_reader_keep_prompts(*args, **kwargs):
             reader = prepare_reader(*args, **kwargs)

Misc/NEWS.d/next/Library/2026-10-17-08-49-47.gh-issue-159617.tWKOER.rst

@@ -0,0 +1,3 @@
+Fix the :term:`REPL` dropping the last characters of a long line that wraps
+across several rows, which could raise :exc:`IndexError` when moving the
+cursor with the arrow keys.