floating: add position reference element override

src/lib/floating.svelte.ts

@@ -41,6 +41,7 @@ export class Popover {
 	};
 	private referenceElement: HTMLElement | undefined = $state();
 	private floatingElement: HTMLElement | undefined = $state();
+	private positionElement: HTMLElement | undefined = $state();
 	private open = $state(false);
 
 	/**
@@ -95,6 +96,23 @@ export class Popover {
 		return attrs;
 	}
 
+	/**
+	 * Optionally sets the positioning element for the popover, which is used
+	 * for calculating the position of the floating element. If not set, the
+	 * reference element is used by default.
+	 * @returns An object containing an attachment key for the positioning element.
+	 */
+	positionReference() {
+		return {
+			[createAttachmentKey()]: (node: HTMLElement) => {
+				this.positionElement = node;
+				return () => {
+					this.positionElement = undefined;
+				};
+			}
+		};
+	}
+
 	/** Returns whether the popover is open */
 	isOpen() {
 		return this.open;
@@ -171,7 +189,8 @@ export class Popover {
 		if (!this.referenceElement || !this.floatingElement) {
 			return;
 		}
-		const position = await computePosition(this.referenceElement, this.floatingElement, {
+		const reference = this.positionElement || this.referenceElement;
+		const position = await computePosition(reference, this.floatingElement, {
 			placement: this.options.placement,
 			middleware: [
 				flip(),