Fr fix

src/textual/_resolve.py

@@ -196,23 +196,30 @@ def resolve_box_models(
     Returns:
         List of resolved box models.
     """
-    margin_width, margin_height = margin
-
-    fraction_width = Fraction(max(0, size.width - margin_width))
-    fraction_height = Fraction(max(0, size.height - margin_height))
 
+    margin_width, margin_height = margin
+    fraction_width = Fraction(size.width)
+    fraction_height = Fraction(size.height)
+    fraction_zero = Fraction(0)
     margin_size = size - margin
 
+    margins = [widget.styles.margin.totals for widget in widgets]
+
     # Fixed box models
     box_models: list[BoxModel | None] = [
         (
             None
             if _dimension is not None and _dimension.is_fraction
             else widget._get_box_model(
-                size, viewport_size, fraction_width, fraction_height
+                size,
+                viewport_size,
+                max(fraction_zero, fraction_width - margin_width),
+                max(fraction_zero, fraction_height - margin_height),
             )
         )
-        for (_dimension, widget) in zip(dimensions, widgets)
+        for (_dimension, widget, (margin_width, margin_height)) in zip(
+            dimensions, widgets, margins
+        )
     ]
 
     if None not in box_models:

tests/snapshot_tests/test_snapshots.py

@@ -8,7 +8,7 @@
 from textual import events, on
 from textual.app import App, ComposeResult
 from textual.binding import Binding, Keymap
-from textual.containers import Center, Grid, Middle, Vertical, VerticalScroll
+from textual.containers import Center, Container, Grid, Middle, Vertical, VerticalScroll
 from textual.pilot import Pilot
 from textual.renderables.gradient import LinearGradient
 from textual.screen import ModalScreen, Screen
@@ -2336,3 +2336,41 @@ def compose(self) -> ComposeResult:
                 yield Label("100%")
 
     assert snap_compare(BackgroundTintApp())
+
+
+def test_fr_and_margin(snap_compare):
+    """Regression test for https://github.com/Textualize/textual/issues/5116"""
+
+    # Check margins can be independently applied to widgets with fr unites
+
+    class FRApp(App):
+        CSS = """
+        #first-container {            
+            background: green;
+            height: auto;
+        }
+
+        #second-container {
+            margin: 2;
+            background: red;
+            height: auto;        
+        }
+
+        #third-container {
+            margin: 4;
+            background: blue;
+            height: auto;
+        }
+        """
+
+        def compose(self) -> ComposeResult:
+            with Container(id="first-container"):
+                yield Label("No margin - should extend to left and right")
+
+            with Container(id="second-container"):
+                yield Label("A margin of 2, should be 2 cells around the text")
+
+            with Container(id="third-container"):
+                yield Label("A margin of 4, should be 4 cells around the text")
+
+    assert snap_compare(FRApp())