merge

sandbox/will/fill.css

@@ -0,0 +1,8 @@
+App Static {
+    border: heavy white;
+    background: blue;
+    color: white;
+    height: 100%;
+
+    box-sizing: border-box;
+}

sandbox/will/fill.py

@@ -0,0 +1,10 @@
+from textual.app import App, ComposeResult
+from textual.widgets import Static
+
+
+class FillApp(App):
+    def compose(self) -> ComposeResult:
+        yield Static("Hello")
+
+
+app = FillApp(css_path="fill.css")

src/textual/box_model.py

@@ -67,7 +67,7 @@ def get_box_model(
         content_width = styles_width.resolve_dimension(
             sizing_container - styles.margin.totals, viewport, fraction_unit
         )
-        if is_border_box and not styles_width.is_percent:
+        if is_border_box and styles_width.excludes_border:
             content_width -= gutter.width
 
     if styles.min_width is not None:
@@ -99,7 +99,7 @@ def get_box_model(
         content_height = styles_height.resolve_dimension(
             sizing_container - styles.margin.totals, viewport, fraction_unit
         )
-        if is_border_box and not styles_height.is_percent:
+        if is_border_box and styles_height.excludes_border:
             content_height -= gutter.height
 
     if styles.min_height is not None:

src/textual/css/scalar.py

@@ -37,6 +37,8 @@ class Unit(Enum):
     AUTO = 8
 
 
+UNIT_EXCLUDES_BORDER = {Unit.CELLS, Unit.FRACTION, Unit.VIEW_WIDTH, Unit.VIEW_HEIGHT}
+
 UNIT_SYMBOL = {
     Unit.CELLS: "",
     Unit.FRACTION: "fr",
@@ -204,6 +206,10 @@ class Scalar(NamedTuple):
         """Check if the unit is a fraction."""
         return self.unit == Unit.FRACTION
 
+    @property
+    def excludes_border(self) -> bool:
+        return self.unit in UNIT_EXCLUDES_BORDER
+
     @property
     def cells(self) -> int | None:
         """Check if the unit is explicit cells."""