Merge pull request #937 from Textualize/align-fix

fix alignment bug

sandbox/will/scrollbug.css

@@ -0,0 +1,9 @@
+Screen {
+    align: center middle;
+}
+
+#test {
+    border: solid white;
+    background: blue;
+}
+

sandbox/will/scrollbug.py

@@ -0,0 +1,19 @@
+from textual.app import App, ComposeResult
+from textual.containers import Container
+from textual.widgets import Header, Footer
+
+
+class ScrollApp(App):
+
+    BINDINGS = [("q", "quit", "QUIT")]
+    CSS_PATH = "scrollbug.css"
+
+    def compose(self) -> ComposeResult:
+        yield Header()
+        yield Container(id="test")
+        yield Footer()
+
+
+app = ScrollApp()
+if __name__ == "__main__":
+    app.run()

src/textual/_arrange.py

@@ -114,7 +114,9 @@ def arrange(
                         for placement in layout_placements
                     ]
                 ).size
-                placement_offset += styles._align_size(placement_size, size).clamped
+                placement_offset += styles._align_size(
+                    placement_size, region.size
+                ).clamped
 
             if placement_offset:
                 layout_placements = [