With the adjustments of `btn-transparent` in https://github.com/discourse/discourse/pull/24666, there are more buttons that could use this class instead of `btn-flat`. This mostly relates to `x` close buttons, but also includes composer and chat toggles.
The primary difference between these styles is that `btn-transparent` never has a background, where `btn-flat` may have a hover or focus background.
1. Use `this.` instead of `{{action}}` where applicable
2. Use `{{fn}}` instead of `@actionParam` where applicable
3. Use non-`@` versions of class/type/tabindex/aria-controls/aria-expanded
4. Remove `btn` class (it's added automatically to all DButtons)
5. Remove `type="button"` (it's the default)
6. Use `concat-class` helper