gh-124927: Fix conversion issue between coordinates and position in REPL

Author: ImFeH2

Lib/_pyrepl/reader.py

@@ -62,7 +62,7 @@ def disp_str(buffer: str) -> tuple[str, list[int]]:
         elif unicodedata.category(c).startswith("C"):
             c = r"\u%04x" % ord(c)
             s.append(c)
-            b.append(len(c) - 1)
+            b.append(len(c))
         else:
             s.append(c)
             b.append(str_width(c))
@@ -577,7 +577,7 @@ def setpos_from_xy(self, x: int, y: int) -> None:
         cur_x = self.screeninfo[i][0]
         while cur_x < x:
             if self.screeninfo[i][1][j] == 0:
-                j += 1
+                j += 1  # prevent infinite loop
                 continue
             cur_x += self.screeninfo[i][1][j]
             j += 1

Lib/test/test_pyrepl/test_reader.py

@@ -313,18 +313,17 @@ def test_key_press_on_tab_press_once(self):
 
         self.assert_screen_equals(reader, f"{code}a")
 
+    def test_pos2xy_with_no_columns(self):
+        console = prepare_console([])
+        reader = prepare_reader(console)
+        # Simulate a resize to 0 columns
+        reader.screeninfo = []
+        self.assertEqual(reader.pos2xy(), (0, 0))
+
     def test_setpos_from_xy_for_non_printing_char(self):
         code = "# non \u200c printing character"
         events = code_to_events(code)
 
         reader, _ = handle_all_events(events)
         reader.setpos_from_xy(8, 0)
-
         self.assertEqual(reader.pos, 7)
-
-    def test_pos2xy_with_no_columns(self):
-        console = prepare_console([])
-        reader = prepare_reader(console)
-        # Simulate a resize to 0 columns
-        reader.screeninfo = []
-        self.assertEqual(reader.pos2xy(), (0, 0))