added expand button part for tree item (#893); fixes #890

docs/resources/changelog.md

@@ -15,6 +15,7 @@ This release removes the `<sl-responsive-media>` component. When this component
 - 🚨 BREAKING: Removed `<sl-responsive-media>` (use the well-supported `aspect-ratio` CSS property instead)
 - Added an expand/collapse animation to `<sl-tree-item>`
 - Added `sl-lazy-change` event to `<sl-tree-item>`
+- Added `expand-button` css part to `<sl-tree-item>` [#890](https://github.com/shoelace-style/shoelace/issues/890)
 - Fixed a bug in `<sl-popup>` that didn't account for the arrow's diagonal size
 - Fixed a bug in `<sl-popup>` that caused arrow placement to be incorrect with RTL
 - Fixed a bug in `<sl-progress-ring>` that caused the indeterminate animation to stop working in Safari [#891](https://github.com/shoelace-style/shoelace/issues/891)

src/components/tree-item/tree-item.ts

@@ -41,6 +41,7 @@ export function isTreeItem(element: Element) {
  * @slot collapse-icon - The icon to show when the item is collapsed.
  *
  * @csspart base - The component's internal wrapper.
+ * @csspart expand-button - The item's expand button.
  * @csspart item - The item main container.
  * @csspart item--disabled - Applied when the item is disabled.
  * @csspart item--expanded - Applied when the item is expanded.
@@ -242,6 +243,7 @@ export default class SlTreeItem extends ShoelaceElement {
               'tree-item__expand-button': true,
               'tree-item__expand-button--visible': showExpandButton
             })}
+            part="expand-button"
             aria-hidden="true"
           >
             ${when(this.loading, () => html` <sl-spinner></sl-spinner> `)}