Respect completions display style in terminal debugger

Make the debugger prompt use the same completions style as the main
IPython prompt.

Closes gh-9746

IPython/terminal/debugger.py

@@ -5,11 +5,14 b' from IPython.core.debugger import Pdb'
 
 from IPython.core.completer import IPCompleter
 from .ptutils import IPythonPTCompleter
-from .shortcuts import suspend_to_bg
+from .shortcuts import suspend_to_bg, cursor_in_leading_ws
 
-from prompt_toolkit.filters import Condition
+from prompt_toolkit.enums import DEFAULT_BUFFER
+from prompt_toolkit.filters import (Condition, HasFocus, HasSelection,
+    ViInsertMode, EmacsInsertMode)
 from prompt_toolkit.keys import Keys
 from prompt_toolkit.key_binding.manager import KeyBindingManager
+from prompt_toolkit.key_binding.bindings.completion import display_completions_like_readline
 from prompt_toolkit.token import Token
 from prompt_toolkit.shortcuts import create_prompt_application
 from prompt_toolkit.interface import CommandLineInterface
@@ -42,6 +45,15 b' class TerminalPdb(Pdb):'
         kbmanager.registry.add_binding(Keys.ControlZ, filter=supports_suspend
                                       )(suspend_to_bg)
 
+        if self.shell.display_completions == 'readlinelike':
+            kbmanager.registry.add_binding(Keys.ControlI,
+                                 filter=(HasFocus(DEFAULT_BUFFER)
+                                         & ~HasSelection()
+                                         & ViInsertMode() | EmacsInsertMode()
+                                         & ~cursor_in_leading_ws
+                                         ))(display_completions_like_readline)
+        multicolumn = (self.shell.display_completions == 'multicolumn')
+
         self._pt_app = create_prompt_application(
                             editing_mode=getattr(EditingMode, self.shell.editing_mode.upper()),
                             key_bindings_registry=kbmanager.registry,
@@ -49,7 +61,8 b' class TerminalPdb(Pdb):'
                             completer= self._ptcomp,
                             enable_history_search=True,
                             mouse_support=self.shell.mouse_support,
-                            get_prompt_tokens=get_prompt_tokens
+                            get_prompt_tokens=get_prompt_tokens,
+                            display_completions_in_columns=multicolumn,
         )
         self.pt_cli = CommandLineInterface(self._pt_app, eventloop=self.shell._eventloop)