check for non-existant rules

src/textual/box_model.py

@@ -54,11 +54,11 @@ def get_box_model(
     else:
         width = max(0, width - styles.margin.width)
 
-    if styles.min_width:
+    if has_rule("min_width"):
         min_width = styles.min_width.resolve_dimension(container_size, parent_size)
         width = max(width, min_width)
 
-    if styles.max_width:
+    if has_rule("max_width"):
         max_width = styles.max_width.resolve_dimension(container_size, parent_size)
         width = min(width, max_width)
 
@@ -70,11 +70,11 @@ def get_box_model(
     else:
         height = max(0, height - styles.margin.height)
 
-    if styles.min_height:
+    if has_rule("min_height"):
         min_height = styles.min_height.resolve_dimension(container_size, parent_size)
         height = max(height, min_height)
 
-    if styles.max_height:
+    if has_rule("max_height"):
         max_height = styles.max_height.resolve_dimension(container_size, parent_size)
         height = min(width, max_height)
 

src/textual/css/styles.py

@@ -132,6 +132,7 @@ class RulesMap(TypedDict, total=False):
 
 
 RULE_NAMES = list(RulesMap.__annotations__.keys())
+RULE_NAMES_SET = frozenset(RULE_NAMES)
 _rule_getter = attrgetter(*RULE_NAMES)
 
 
@@ -416,6 +417,7 @@ class Styles(StylesBase):
         return Styles(node=self.node, _rules=self.get_rules(), important=self.important)
 
     def has_rule(self, rule: str) -> bool:
+        assert rule in RULE_NAMES_SET, f"no such rule {rule!r}"
         return rule in self._rules
 
     def clear_rule(self, rule: str) -> bool:

tests/test_geometry.py

@@ -317,6 +317,13 @@ def test_spacing_add():
         Spacing(1, 2, 3, 4) + "foo"
 
 
+def test_spacing_sub():
+    assert Spacing(1, 2, 3, 4) - Spacing(5, 6, 7, 8) == Spacing(-4, -4, -4, -4)
+
+    with pytest.raises(TypeError):
+        Spacing(1, 2, 3, 4) - "foo"
+
+
 def test_split():
     assert Region(10, 5, 22, 15).split(10, 5) == (
         Region(10, 5, 10, 5),