Allow nested scrolling of Windows

These changes mean that Windows will return NotImplemented when attempting to
scroll beyond their scrollable limits, allowing mouse scroll events to bubble
up to a parent container when a window in a scrollable container is scrolled.

src/prompt_toolkit/layout/containers.py

@@ -2551,33 +2551,33 @@ def _mouse_handler(self, mouse_event: MouseEvent) -> NotImplementedOrNone:
         key binding (no UI invalidate required in that case).
         """
         if mouse_event.event_type == MouseEventType.SCROLL_DOWN:
-            self._scroll_down()
-            return None
+            return self._scroll_down()
         elif mouse_event.event_type == MouseEventType.SCROLL_UP:
-            self._scroll_up()
-            return None
+            return self._scroll_up()
 
         return NotImplemented
 
-    def _scroll_down(self) -> None:
+    def _scroll_down(self) -> "NotImplementedOrNone":
         "Scroll window down."
         info = self.render_info
 
         if info is None:
-            return
+            return NotImplemented
 
         if self.vertical_scroll < info.content_height - info.window_height:
             if info.cursor_position.y <= info.configured_scroll_offsets.top:
                 self.content.move_cursor_down()
-
             self.vertical_scroll += 1
+            return None
+
+        return NotImplemented
 
-    def _scroll_up(self) -> None:
+    def _scroll_up(self) -> "NotImplementedOrNone":
         "Scroll window up."
         info = self.render_info
 
         if info is None:
-            return
+            return NotImplemented
 
         if info.vertical_scroll > 0:
             # TODO: not entirely correct yet in case of line wrapping and long lines.
@@ -2586,8 +2586,10 @@ def _scroll_up(self) -> None:
                 >= info.window_height - 1 - info.configured_scroll_offsets.bottom
             ):
                 self.content.move_cursor_up()
-
             self.vertical_scroll -= 1
+            return None
+
+        return NotImplemented
 
     def get_key_bindings(self) -> KeyBindingsBase | None:
         return self.content.get_key_bindings()

src/prompt_toolkit/layout/controls.py

@@ -477,9 +477,7 @@ def create_content(self, width: int, height: int) -> UIContent:
         def get_line(i: int) -> StyleAndTextTuples:
             return []
 
-        return UIContent(
-            get_line=get_line, line_count=100**100
-        )  # Something very big.
+        return UIContent(get_line=get_line, line_count=1)
 
     def is_focusable(self) -> bool:
         return False