gh-118878: pyrepl: Show completion menu below current line

The main advantage of this is that the behaviour is less janky,
since the current line no longer jumps up and down.

This also allows fixing the behaviour of arrow keys when the menu is
displayed.

Lib/_pyrepl/commands.py

@@ -276,7 +276,10 @@ def do(self) -> None:
             x, y = r.pos2xy()
             new_y = y + 1
 
-            if new_y > r.max_row():
+            # bol1 = r.bol()
+            eol1 = r.eol()
+            if eol1 == len(b):
+            #if new_y > r.max_row():
                 if r.historyi < len(r.history):
                     r.select_item(r.historyi + 1)
                     r.pos = r.eol(0)
@@ -303,7 +306,7 @@ def do(self) -> None:
 class left(MotionCommand):
     def do(self) -> None:
         r = self.reader
-        for i in range(r.get_arg()):
+        for _ in range(r.get_arg()):
             p = r.pos - 1
             if p >= 0:
                 r.pos = p
@@ -315,7 +318,7 @@ class right(MotionCommand):
     def do(self) -> None:
         r = self.reader
         b = r.buffer
-        for i in range(r.get_arg()):
+        for _ in range(r.get_arg()):
             p = r.pos + 1
             if p <= len(b):
                 r.pos = p

Lib/_pyrepl/completing_reader.py

@@ -30,7 +30,7 @@
 # types
 Command = commands.Command
 if False:
-    from .types import Callback, SimpleContextManager, KeySpec, CommandName
+    from .types import KeySpec, CommandName
 
 
 def prefix(wordlist: list[str], j: int = 0) -> str:
@@ -258,10 +258,14 @@ def after_command(self, cmd: Command) -> None:
     def calc_screen(self) -> list[str]:
         screen = super().calc_screen()
         if self.cmpltn_menu_vis:
-            ly = self.lxy[1]
+            ly = self.lxy[1] + 1
             screen[ly:ly] = self.cmpltn_menu
-            self.screeninfo[ly:ly] = [(0, [])]*len(self.cmpltn_menu)
-            self.cxy = self.cxy[0], self.cxy[1] + len(self.cmpltn_menu)
+            # This is a horrible hack. If we're not in the middle
+            # of multiline edit, don't append to screeninfo
+            # since that screws up the position calculation
+            # in pos2xy function.
+            if self.pos != len(self.buffer):
+                self.screeninfo[ly:ly] = [(0, [])]*len(self.cmpltn_menu)
         return screen
 
     def finish(self) -> None:

Lib/test/test_pyrepl.py

@@ -617,6 +617,7 @@ def test_updown_arrow_with_completion_menu(self):
         events = itertools.chain(
             code_to_events(code),
             [
+                Event(evt="key", data="down", raw=bytearray(b"\x1bOB")),
                 Event(evt='key', data='up', raw=bytearray(b'\x1bOA')),
                 Event(evt="key", data="down", raw=bytearray(b"\x1bOB")),
             ],