class_docstr: Two dimensional raster of callbacks for mouse events.
code: def __init__(self) -> None:
      def dummy_callback(
          mouse_event: MouseEvent,
      ) -> NotImplementedOrNone:
          """
          :param mouse_event: `MouseEvent` instance.
          """
          return NotImplemented
  
      # NOTE: Previously, the data structure was a dictionary mapping (x,y)
      # to the handlers. This however would be more inefficient when copying
      # over the mouse handlers of the visible region in the scrollable pane.
  
      # Map y (row) to x (column) to handlers.
      self.mouse_handlers: defaultdict[
          int, defaultdict[int, MouseHandler]
      ] = defaultdict(lambda: defaultdict(lambda: dummy_callback))
  
imports: from __future__ import annotations
  from collections import defaultdict
  from typing import TYPE_CHECKING, Callable
  from prompt_toolkit.mouse_events import MouseEvent
label_desc: Use image node_id 1 to create a new MouseHandlers object  with example: obj = MouseHandlers()