more docs

docs/styles/box_sizing.md

@@ -0,0 +1,42 @@
+# Box-sizing
+
+The `box-sizing` rule impacts how `width` and `height` rules are translated in to screen dimensions, when combined with `padding` and `border`.
+
+The default value is `border-box` which means that padding and border are included in in the width and height. This setting means that if you add padding and/or border the widget will not change in size, but you will have less space for content.
+
+You can set `box-sizing` to `content-box` which tells Textual that padding and border should increase the size of the widget, leaving the content area unaffected.
+
+## Example
+
+Both widgets in this example have the same height (5). The top widget has `box-sizing: border-box` which means that padding and border reduces the space for content. The bottom widget has `box-sizing: content-box` which increases the size of the widget to compensate for padding and border.
+
+=== "box_sizing.py"
+
+    ```python
+    --8<-- "docs/examples/styles/box_sizing.py"
+    ```
+
+=== "Output"
+
+    ```{.textual path="docs/examples/styles/box_sizing.py"}
+    ```
+
+## CSS
+
+```sass
+/* Set box sizing to border-box (default) */
+box-sizing: border-box;
+
+/* Set box sizing to content-box */
+box-sizing: content-box;
+```
+
+## Python
+
+```python
+# Set box sizing to border-box (default)
+widget.box_sizing = "border-box"
+
+# Set box sizing to content-box
+widget.box_sizing = "content-box"
+```