fix: fix alignment with auto and min height/width

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
 
+### Fixed
+
+- Fixed issue with alignment in containers with auto and min height/width https://github.com/Textualize/textual/issues/5608
+
 ### Added
 
 - Added Widget.preflight_checks to perform some debug checks after a widget is instantiated, to catch common errors. https://github.com/Textualize/textual/pull/5588

src/textual/_arrange.py

@@ -100,12 +100,34 @@ def arrange(
             if styles.align_horizontal != "left" or styles.align_vertical != "top":
                 bounding_region = WidgetPlacement.get_bounds(layout_placements)
                 container_width, container_height = dock_region.size
+
+                align_width = container_width
+                if styles.is_auto_width:
+                    align_width = (
+                        int(
+                            styles.min_width.resolve(
+                                size, viewport, Fraction(container_width)
+                            )
+                        )
+                        if styles.min_width
+                        else 0
+                    )
+
+                align_height = container_height
+                if styles.is_auto_height:
+                    align_height = (
+                        int(
+                            styles.min_height.resolve(
+                                size, viewport, Fraction(container_height)
+                            )
+                        )
+                        if styles.min_height
+                        else 0
+                    )
+
                 placement_offset += styles._align_size(
                     bounding_region.size,
-                    Size(
-                        0 if styles.is_auto_width else container_width,
-                        0 if styles.is_auto_height else container_height,
-                    ),
+                    Size(align_width, align_height),
                 ).clamped
 
             if placement_offset:

tests/snapshot_tests/test_snapshots.py

@@ -3767,3 +3767,34 @@ def on_mount(self) -> None:
                 label.border_title = "Border title"
 
     assert snap_compare(BorderTitleApp())
+
+
+def test_alignment_with_auto_and_min_height(snap_compare):
+    """Regression test for https://github.com/Textualize/textual/issues/5608
+
+    You should see a blue label that is centered both horizontally and vertically
+    within a pink container. The container has auto width and height, but also
+    a min-width of 20 and a min-height of 3.
+    """
+
+    class AlignmentApp(App):
+        CSS = """
+        Container {
+            align: center middle;
+            height: auto;
+            min-height: 3;
+            width: auto;
+            min-width: 20;
+            background: pink;
+        }
+
+        Label {
+            background: blue;
+        }
+        """
+
+        def compose(self) -> ComposeResult:
+            with Container():
+                yield Label("centered")
+
+    assert(snap_compare(AlignmentApp()))