Expanding fr (#2221)

* forced fr to expand

* margin size

* remove comment

* missing snapshot

* snapshot tests

* changelog

* optimize

* snapshot fix

* snapshot update

* snapshot and fixes

* docstrings [skip ci]

tests/snapshot_tests/snapshot_apps/auto_fr.py

@@ -0,0 +1,40 @@
+from textual.app import App, ComposeResult
+from textual.widget import Widget
+from textual.widgets import Label
+
+
+class FRApp(App):
+    CSS = """    
+    Screen {
+        align: center middle;
+        border: solid cyan;
+    }
+
+    #container {  
+        width: 30;      
+        height: auto;
+        border: solid green;
+        overflow-y: auto;
+    }
+
+    #child {
+        height: 1fr;
+        border: solid red;       
+    }
+
+    #bottom {
+        margin: 1 2;    
+        background: $primary;
+    }
+    """
+
+    def compose(self) -> ComposeResult:
+        with Widget(id="container"):
+            yield Label("Hello one line", id="top")
+            yield Widget(id="child")
+            yield Label("Two\nLines with 1x2 margin", id="bottom")
+
+
+if __name__ == "__main__":
+    app = FRApp()
+    app.run()

tests/snapshot_tests/snapshot_apps/fr_margins.py

@@ -0,0 +1,35 @@
+from textual.app import App, ComposeResult
+from textual.widgets import Label
+from textual.containers import Container
+
+
+# Test fr dimensions and margins work in an auto container
+# https://github.com/Textualize/textual/issues/2220
+class TestApp(App):
+    CSS = """
+    Container {
+        background: green 20%;
+        border: heavy green;
+        width: auto;
+        height: auto;
+        overflow: hidden;
+    }
+
+    Label {
+        background: green 20%;      
+        width: 1fr;
+        height: 1fr;
+        margin: 2 2;            
+    }
+    """
+
+    def compose(self) -> ComposeResult:
+        with Container():
+            yield Label("Hello")
+            yield Label("World")
+            yield Label("!!")
+
+
+if __name__ == "__main__":
+    app = TestApp()
+    app.run()

tests/snapshot_tests/test_snapshots.py

@@ -388,6 +388,16 @@ def test_dock_scroll(snap_compare):
     assert snap_compare(SNAPSHOT_APPS_DIR / "dock_scroll.py", terminal_size=(80, 25))
 
 
+def test_auto_fr(snap_compare):
+    # https://github.com/Textualize/textual/issues/2220
+    assert snap_compare(SNAPSHOT_APPS_DIR / "auto_fr.py", terminal_size=(80, 25))
+
+
+def test_fr_margins(snap_compare):
+    # https://github.com/Textualize/textual/issues/2220
+    assert snap_compare(SNAPSHOT_APPS_DIR / "fr_margins.py", terminal_size=(80, 25))
+
+
 def test_scroll_visible(snap_compare):
     # https://github.com/Textualize/textual/issues/2181
     assert snap_compare(SNAPSHOT_APPS_DIR / "scroll_visible.py", press=["t"])