Fix 'style panel doesn't always disappear if you switch to the hand/laser tools' (#2886)

We had some bad logic in `useRelevantStyles` explicitly allowing an
opacity-slider to be rendered at all times when there is at least one
shape selected.

This shouldn't be the case when the editor is in non-shape-focused tools
like the move tool and the laser pointer tool. I refactored the hook
slightly to make it easier to express the correct logic. See the comment
for a more detailed description.

### Change Type

- [x] `patch` — Bug fix


[^1]: publishes a `patch` release, for devDependencies use `internal`
[^2]: will not publish a new version


### Release Notes

- Fixes an bug causing the opacity slider to show up in the move tool
and laser pointer tool.

packages/editor/src/lib/editor/Editor.ts

@@ -7168,19 +7168,17 @@ export class Editor extends EventEmitter<TLEventMap> {
 	 *
 	 * @internal
 	 */
-	private _selectionSharedStyles = computed<ReadonlySharedStyleMap>(
-		'_selectionSharedStyles',
-		() => {
-			const selectedShapes = this.getSelectedShapes()
+	@computed
+	private _getSelectionSharedStyles(): ReadonlySharedStyleMap {
+		const selectedShapes = this.getSelectedShapes()
 
-			const sharedStyles = new SharedStyleMap()
-			for (const selectedShape of selectedShapes) {
-				this._extractSharedStyles(selectedShape, sharedStyles)
-			}
-
-			return sharedStyles
+		const sharedStyles = new SharedStyleMap()
+		for (const selectedShape of selectedShapes) {
+			this._extractSharedStyles(selectedShape, sharedStyles)
 		}
-	)
+
+		return sharedStyles
+	}
 
 	/** @internal */
 	getStyleForNextShape<T>(style: StyleProp<T>): T {
@@ -7213,7 +7211,7 @@ export class Editor extends EventEmitter<TLEventMap> {
 		// If we're in selecting and if we have a selection, return the shared styles from the
 		// current selection
 		if (this.isIn('select') && this.getSelectedShapeIds().length > 0) {
-			return this._selectionSharedStyles.get()
+			return this._getSelectionSharedStyles()
 		}
 
 		// If the current tool is associated with a shape, return the styles for that shape.