Revert d92c252ea6

For now anyway. I suspect this definition if is_scrollable will become moot
soon (ideally there would never be an is_scrollable property at all, ever,
as inheritance should ideally dictate this if the changes that are planned
go the way as planned -- a property that tells you something about
provenance when the inheritance tree tells you that is some bad OO smell)
but I want to get the tests set up as their own PR first and *then* work on
the fix.

src/textual/widget.py

@@ -1227,7 +1227,7 @@ class Widget(DOMNode):
         Returns:
             bool: True if this widget may be scrolled.
         """
-        return self.is_container
+        return self.styles.layout is not None or bool(self.children)
 
     @property
     def layer(self) -> str: