Fix for auto height in collapsible

Author: willmcgugan

src/textual/layout.py

@@ -261,9 +261,8 @@ def get_content_height(
                 child.styles.is_dynamic_height for child in widget.displayed_children
             ):
                 # An exception for containers with all dynamic height widgets
-                arrangement = widget._arrange(
-                    Size(width, container.height - widget.gutter.height)
-                )
+                arrangement = widget._arrange(Size(width, container.height))
+                return arrangement.total_region.bottom
             else:
                 arrangement = widget._arrange(Size(width, 0))
             height = arrangement.total_region.bottom

src/textual/widget.py

@@ -1590,7 +1590,11 @@ def _get_box_model(
         elif is_auto_height:
             # Calculate dimensions based on content
             content_height = Fraction(
-                self.get_content_height(content_container, viewport, int(content_width))
+                self.get_content_height(
+                    content_container - margin.totals,
+                    viewport,
+                    int(content_width),
+                )
             )
             if (
                 styles.overflow_y == "auto" and styles.scrollbar_gutter == "stable"

tests/snapshot_tests/test_snapshots.py

@@ -3285,7 +3285,11 @@ def test_collapsible_datatable(snap_compare):
     class MyApp(App):
         CSS = """
         DataTable {
-        
+            
+        }
+        Collapsible {
+            max-height: 50%;
+            # height: 1fr;
         }
         """
 
@@ -3294,8 +3298,8 @@ def compose(self) -> ComposeResult:
             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%"
+            # 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"