Add help text for offset-x and offset-y properies

sandbox/uber.css

@@ -8,5 +8,4 @@
 .list-item {
   height: 8;
   background: dark_blue;
-  offset: 12 1;
 }

sandbox/uber.py

@@ -80,7 +80,8 @@ class BasicApp(App):
         self.focused.display = not self.focused.display
 
     def action_toggle_border(self):
-        self.focused.styles.border = [("solid", "red"), ("dashed", "white")]
+        # self.focused.styles.border = [("solid", "red"), ("dashed", "white")]
+        self.focused.styles.offset = (12, "1x")
 
 
 BasicApp.run(css_file="uber.css", log="textual.log", log_verbosity=1)

src/textual/css/_help_text.py

@@ -610,3 +610,19 @@ def align_help_text() -> HelpText:
             ),
         ],
     )
+
+
+def offset_single_axis_help_text(property_name: str) -> HelpText:
+    return HelpText(
+        summary=f"Invalid value for [i]{property_name}[/]",
+        bullets=[
+            Bullet(
+                markup=f"The [i]{property_name}[/] property can be set to a number or scalar value",
+                examples=[
+                    Example(f"{property_name}: 10;"),
+                    Example(f"{property_name}: 50%;"),
+                ],
+            ),
+            Bullet(f"Valid scalar units are {friendly_list(SYMBOL_UNIT)}"),
+        ],
+    )

src/textual/css/_style_properties.py

@@ -18,6 +18,7 @@ from ._help_text import (
     border_property_help_text,
     layout_property_help_text,
     fractional_property_help_text,
+    offset_property_help_text,
 )
 from ._help_text import (
     spacing_wrong_number_of_values,
@@ -591,16 +592,23 @@ class OffsetProperty:
                 obj.refresh(layout=True)
         else:
             x, y = offset
-            scalar_x = (
-                Scalar.parse(x, Unit.WIDTH)
-                if isinstance(x, str)
-                else Scalar(float(x), Unit.CELLS, Unit.WIDTH)
-            )
-            scalar_y = (
-                Scalar.parse(y, Unit.HEIGHT)
-                if isinstance(y, str)
-                else Scalar(float(y), Unit.CELLS, Unit.HEIGHT)
-            )
+
+            try:
+                scalar_x = (
+                    Scalar.parse(x, Unit.WIDTH)
+                    if isinstance(x, str)
+                    else Scalar(float(x), Unit.CELLS, Unit.WIDTH)
+                )
+                scalar_y = (
+                    Scalar.parse(y, Unit.HEIGHT)
+                    if isinstance(y, str)
+                    else Scalar(float(y), Unit.CELLS, Unit.HEIGHT)
+                )
+            except ScalarParseError as error:
+                raise StyleValueError(
+                    str(error), help_text=offset_property_help_text(context="inline")
+                )
+
             _offset = ScalarOffset(scalar_x, scalar_y)
 
             if obj.set_rule(self.name, _offset):

src/textual/css/_styles_builder.py

@@ -19,6 +19,7 @@ from ._help_text import (
     fractional_property_help_text,
     align_help_text,
     offset_property_help_text,
+    offset_single_axis_help_text,
 )
 from .constants import (
     VALID_ALIGN_HORIZONTAL,
@@ -476,11 +477,11 @@ class StylesBuilder:
         if not tokens:
             return
         if len(tokens) != 1:
-            self.error(name, tokens[0], f"expected a single number")
+            self.error(name, tokens[0], offset_single_axis_help_text(name))
         else:
             token = tokens[0]
             if token.name not in ("scalar", "number"):
-                self.error(name, token, f"expected a scalar; found {token.value!r}")
+                self.error(name, token, offset_single_axis_help_text(name))
             x = Scalar.parse(token.value, Unit.WIDTH)
             y = self.styles.offset.y
             self.styles._rules["offset"] = ScalarOffset(x, y)
@@ -489,11 +490,11 @@ class StylesBuilder:
         if not tokens:
             return
         if len(tokens) != 1:
-            self.error(name, tokens[0], f"expected a single number")
+            self.error(name, tokens[0], offset_single_axis_help_text(name))
         else:
             token = tokens[0]
             if token.name not in ("scalar", "number"):
-                self.error(name, token, f"expected a scalar; found {token.value!r}")
+                self.error(name, token, offset_single_axis_help_text(name))
             y = Scalar.parse(token.value, Unit.HEIGHT)
             x = self.styles.offset.x
             self.styles._rules["offset"] = ScalarOffset(x, y)

src/textual/layouts/grid.py

@@ -245,9 +245,6 @@ class GridLayout(Layout):
         col_align: GridAlign,
         row_align: GridAlign,
     ) -> Region:
-        offset_x = 0
-        offset_y = 0
-
         def align(size: int, container: int, align: GridAlign) -> int:
             offset = 0
             if align == "end":