proposal: Border widths and radiuses

text/0016-borders-widths-and-radiuses.md

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+# Border Widths and Radiuses
+
+## Summary
+
+The ability to separately define the width of each side, and radius of each corner.
+
+## Motivation
+
+There are instances where a theme for a desktop GUI toolkit will require to specify a separate border width or radius. Such as a collection of conjoined buttons where only one side has a radius, or only the edges have a radius.
+
+Designs for libcosmic have a navigation button within the left corner of the headerbar where the button hugs the edge of the window and defines a radius only the top-left and bottom-right corners. And there are some instances where a highlighted label likewise has a radius defined only for one side.
+
+## Guide-level explanation
+
+The border_width and border_radius are defined with a `Border`. Similar to `Padding`, this can be created from either a `f32`, `[f32; 2]`, or `[f32; 4]`. Internally, it is represented as:
+
+```rs
+pub struct Border([f32; 4]);
+```
+
+Which may be constructed with one of the following:
+
+- `Border::from(10.0)`
+- `Border::from([10.0, 5.0])`
+- `Border::from([10.0, 5.0, 10.0, 0.0])`
+
+For the `border_width` parameter, an input of `10.0` would assign a width of `10.0` to all sides. An input of `[10.0, 5.0]` would assign the top and bottom width to `10.0`, and the left and right width to `5.0`. Finally, an input of `[10.0, 5.0, 10.0, 0.0]` would assign widths clockwise starting from the top width of `10.0`, the right width of `5.0`, the bottom width of `10.0`, and the left width of `0.0`.
+
+For the `border_radius` parameter, an input of `10.0` would assign a radius of `10.0` to all corners. An input of `[10.0, 5.0]` would assign the top-left and bottom-right radius to `10.0`, and the bottom-left and top-right radius to `5.0`. Finally, an input of `[10.0, 5.0, 10.0, 0.0]` would assign the radius clockwise starting from the top-left radius of `10.0`, the top-right radius of `5.0`, the bottom-right radius of `10.0`, and the bottom-left radius of `0.0`.
+
+
+## Implementation strategy
+
+- Create a `core::border` module containing a `Border` type that is identical to `Padding`.
+- Update every instance of `border_radius` and `border_width` to use `Border`.
+- Alter renderer to support the new conditional widths and radiuses.
+
+## Drawbacks
+
+Substantial changes required to convert every instance of `border_width` and `border_radius`. Any custom theme implementations would have to switch to the new type wherever they define these parameters.
+
+## Rationale and alternatives
+
+This space is already well explored by CSS. The border_width and border_radius parameters optionally accept one, two, three, or four parameters.
+
+## Prior art
+
+Both CSS and GTK support this functionality. GTK has container widgets which alter the radiuses of its children separately based on their position. Such as the `ButtonBox` container that defines a radius only for the left-most button and the right-most button, with all buttons inbetween having no radius.
+
+## Unresolved questions
+
+None
+
+## Future possibilities
+
+It would be possible to take inspiration from CSS and allow for the border to be defined as `Thin`, `Medium`, and `Thick`. Where the theme could define the values of these presets.