fix prune issue

src/textual/_compositor.py

@@ -614,9 +614,8 @@ def add_widget(
                                 - widget.scrollbar_size_horizontal
                             )
                         )
-                        capped_scroll_y = widget.validate_scroll_y(new_scroll_y)
-                        widget.set_reactive(Widget.scroll_y, capped_scroll_y)
-                        widget.set_reactive(Widget.scroll_target_y, capped_scroll_y)
+                        widget.set_reactive(Widget.scroll_y, new_scroll_y)
+                        widget.set_reactive(Widget.scroll_target_y, new_scroll_y)
                         widget.vertical_scrollbar._reactive_position = new_scroll_y
 
                     if visible_only:

src/textual/widget.py

@@ -4274,9 +4274,7 @@ def remove_children(
             ]
         else:
             children_to_remove = selector
-        await_remove = self.app._prune(
-            *children_to_remove, parent=cast(DOMNode, self._parent)
-        )
+        await_remove = self.app._prune(*children_to_remove, parent=self)
         return await_remove
 
     @asynccontextmanager
@@ -4423,11 +4421,12 @@ def _check_refresh(self) -> None:
                     self.call_later(self._update_styles)
                 if self._scroll_required:
                     self._scroll_required = False
-                    if self.styles.keyline[0] != "none":
-                        # TODO: Feels like a hack
-                        # Perhaps there should be an explicit mechanism for backgrounds to refresh when scrolled?
-                        self._set_dirty()
-                    screen.post_message(messages.UpdateScroll())
+                    if not self._layout_required:
+                        if self.styles.keyline[0] != "none":
+                            # TODO: Feels like a hack
+                            # Perhaps there should be an explicit mechanism for backgrounds to refresh when scrolled?
+                            self._set_dirty()
+                        screen.post_message(messages.UpdateScroll())
                 if self._repaint_required:
                     self._repaint_required = False
                     if self.display:

tests/snapshot_tests/test_snapshots.py

@@ -4713,3 +4713,29 @@ def compose(self) -> ComposeResult:
             yield Static("Hello, World! 293487 " * 200)
 
     assert snap_compare(ScrollbarApp())
+
+
+def test_prune_fix(snap_compare) -> None:
+    """Regression test for https://github.com/Textualize/textual/issues/6205
+
+    You should see the text "Hello" and "World" across the first two lines.
+    The original issue is that a layout operation wasn't done after removing children, leaving
+    a large gap between "Hello" and "World"
+
+    """
+
+    class PruneApp(App):
+        BINDINGS = [Binding("c", "clear", priority=True)]
+
+        def compose(self) -> ComposeResult:
+            yield Static("Hello")
+            with VerticalGroup():
+                for i in range(10):
+                    yield Static(str(i))
+            yield Static("World")
+
+        async def action_clear(self) -> None:
+            vs = self.query_one(VerticalGroup)
+            await vs.remove_children()
+
+    assert snap_compare(PruneApp(), press=["c"])