fix for datatable in collapsible

src/textual/layout.py

@@ -257,7 +257,14 @@ def get_content_height(
             Content height (in lines).
         """
         if widget._nodes:
-            arrangement = widget._arrange(Size(width, 0))
+            if not widget.styles.is_docked and all(
+                child.styles.is_dynamic_height for child in widget.displayed_children
+            ):
+                arrangement = widget._arrange(
+                    Size(width, container.height - widget.gutter.height)
+                )
+            else:
+                arrangement = widget._arrange(Size(width, 0))
             height = arrangement.total_region.bottom
         else:
             height = 0

src/textual/layouts/grid.py

@@ -28,7 +28,9 @@ def arrange(
         parent.pre_layout(self)
         styles = parent.styles
         row_scalars = styles.grid_rows or (
-            [Scalar.parse("1fr")] if size.height else [Scalar.parse("auto")]
+            [Scalar.parse("1fr")]
+            if (size.height and not parent.styles.is_auto_height)
+            else [Scalar.parse("auto")]
         )
         column_scalars = styles.grid_columns or [Scalar.parse("1fr")]
         gutter_horizontal = styles.grid_gutter_horizontal

tests/snapshot_tests/test_snapshots.py

@@ -26,6 +26,7 @@
 from textual.screen import ModalScreen, Screen
 from textual.widgets import (
     Button,
+    Collapsible,
     DataTable,
     Footer,
     Header,
@@ -3251,3 +3252,35 @@ async def run_before(pilot: Pilot) -> None:
         terminal_size=(175, 50),
         run_before=run_before,
     )
+
+
+def test_collapsible_datatable(snap_compare):
+    """Regression test for https://github.com/Textualize/textual/issues/5407
+
+    You should see two collapsibles, where the first is expanded.
+    In the expanded coillapsible, you should see a DataTable filling the space,
+    with all borders and both scrollbars visible.
+    """
+
+    class MyApp(App):
+        CSS = """
+        DataTable {
+        
+        }
+        """
+
+        def compose(self) -> ComposeResult:
+            yield Collapsible(DataTable(id="t1"), id="c1", collapsed=False)
+            yield Collapsible(Label("hello"), id="c2")
+
+        def on_mount(self) -> None:
+            self.query_one("#c1", Collapsible).styles.max_height = "50%"
+            self.query_one("#c2", Collapsible).styles.max_height = "50%"
+
+            t1 = self.query_one("#t1", DataTable)
+            t1.styles.border = "heavy", "black"
+            t1.add_column("A")
+            for number in range(1, 100):
+                t1.add_row(str(number) + " " * 200)
+
+    snap_compare(MyApp())