Terminal: display function signature and docstring popup after opening parentheses - #15412

Closes #12789.

Summary

Implements a signature and docstring popup in terminal IPython when typing an opening parenthesis ( on callables or editing arguments inside function calls, providing a built-in experience similar to ptpython.

Changes

1. IPython/terminal/docstring.py

   • Implements DocstringTooltip using prompt-toolkit layout primitives (FloatContainer, Float(xcursor=True, ycursor=True), Window, and FormattedTextControl).
   • Implements dual-engine introspection: primary via jedi.Interpreter.get_signatures(), with fallback to shell._ofind() and inspect.signature / inspect.getdoc.
   • Runs introspection asynchronously in a thread executor (loop.run_in_executor) to keep typing responsive and non-blocking.
   • Includes debounced scheduling (delay=0.2s default) and a monotonic sequence counter (_request_id) to discard stale background responses when typing quickly.
   • Automatically hides the popup when the autocomplete/completion menu is active (~has_completions) or when prompt evaluation completes.
   • Uses class:completion-menu styling to respect light, dark, neutral, and nocolor themes.

2. IPython/terminal/interactiveshell.py

   • Adds configurable traitlets to TerminalInteractiveShell:

     • display_docstring_popup: Bool(True) to enable/disable the popup.
     • docstring_popup_delay: Float(0.2) to adjust the debounce delay.

   • Attaches DocstringTooltip during init_prompt_toolkit_cli().

   • Ensures docstring_tooltip.clear() is called when prompt input completes.

3. IPython/terminal/tests/test_docstring.py

   • Adds 21 unit and integration tests covering:

     • Builtin functions, user functions, classes, bound methods, and lambdas.
     • Nested function calls and multiline arguments.
     • Definition headers, strings, and closed parentheses.
     • Custom dynamic __signature__ descriptors and non-callable objects.
     • Async debouncing, background task cancellation, and stale-request rejection.
     • Traitlet configuration options.

Verification

• IPython/terminal/tests/test_docstring.py: 21 passed
• IPython/terminal/tests/: 38 passed
• tests/test_oinspect.py tests/test_shortcuts.py: 189 passed, 2 skipped
• tests/test_completer.py: 268 passed, 2 skipped, 4 xfailed
• git diff --check: Clean

Configuration

The popup is enabled by default and can be disabled with:

c.TerminalInteractiveShell.display_docstring_popup = False