Invalidate height map when items are deleted

VirtualList.svelte

@@ -6,8 +6,6 @@
 	export let height = '100%';
 	export let itemHeight = undefined;
 
-	let foo;
-
 	// read-only, but visible to consumers via bind:start
 	export let start = 0;
 	export let end = 0;
@@ -30,103 +28,114 @@
 	});
 
 	// whenever `items` changes, invalidate the current heightmap
-	$: if (mounted) refresh(items, viewport_height, itemHeight);
+	$: if (mounted) invalidateHeightMap(items);
+
+
+  async function invalidateHeightMap(items) {
+
+    if (items.length === 0) {
+      start = 0
+      end = 0
+      return;
+	  top = 0
+      bottom = 0
+      height_map = []
+
+    }
+    if (start > items.length - 1) {
+      start = items.length - 1
+	  top = 0
+      bottom = 0
+      height_map = []
+    }
+    if (end > items.length - 1) {
+      end = items.length - 1
+	  top = 0
+      bottom = 0
+      height_map = []
+    }
+
+	await refresh(items, viewport_height, itemHeight);
+	await tick();
+	await handle_scroll();
+  }
 
 	async function refresh(items, viewport_height, itemHeight) {
+
 		const { scrollTop } = viewport;
 
-		await tick(); // wait until the DOM is up to date
+    	// if items has changed, we have to check to see if start and end are still in range
 
+		await tick(); // wait until the DOM is up to date
 		let content_height = top - scrollTop;
+
 		let i = start;
 
 		while (content_height < viewport_height && i < items.length) {
 			let row = rows[i - start];
-
 			if (!row) {
 				end = i + 1;
 				await tick(); // render the newly visible row
 				row = rows[i - start];
 			}
-
 			const row_height = height_map[i] = itemHeight || row.offsetHeight;
 			content_height += row_height;
 			i += 1;
 		}
 
 		end = i;
-
 		const remaining = items.length - end;
 		average_height = (top + content_height) / end;
-
 		bottom = remaining * average_height;
 		height_map.length = items.length;
-
 	}
 
 	async function handle_scroll() {
 		const { scrollTop } = viewport;
-
 		const old_start = start;
-
 		for (let v = 0; v < rows.length; v += 1) {
 			height_map[start + v] = itemHeight || rows[v].offsetHeight;
 		}
-
 		let i = 0;
 		let y = 0;
-
 		while (i < items.length) {
 			const row_height = height_map[i] || average_height;
 			if (y + row_height > scrollTop) {
 				start = i;
 				top = y;
-
 				break;
 			}
-
 			y += row_height;
 			i += 1;
 		}
-
 		while (i < items.length) {
 			y += height_map[i] || average_height;
 			i += 1;
-
 			if (y > scrollTop + viewport_height) break;
 		}
-
 		end = i;
-
 		const remaining = items.length - end;
 		average_height = y / end;
-
 		while (i < items.length) height_map[i++] = average_height;
 		bottom = remaining * average_height;
-
 		// prevent jumping if we scrolled up into unknown territory
 		if (start < old_start) {
 			await tick();
-
 			let expected_height = 0;
 			let actual_height = 0;
-
 			for (let i = start; i < old_start; i +=1) {
 				if (rows[i - start]) {
 					expected_height += height_map[i];
 					actual_height += itemHeight || rows[i - start].offsetHeight;
 				}
 			}
-
 			const d = actual_height - expected_height;
 			viewport.scrollTo(0, scrollTop + d);
 		}
-
 		// TODO if we overestimated the space these
 		// rows would occupy we may need to add some
 		// more. maybe we can just call handle_scroll again?
 	}
-
 	// trigger initial refresh
 	onMount(() => {
 		rows = contents.getElementsByTagName('svelte-virtual-list-row');
@@ -141,11 +150,9 @@
 		-webkit-overflow-scrolling:touch;
 		display: block;
 	}
-
 	svelte-virtual-list-contents, svelte-virtual-list-row {
 		display: block;
 	}
-
 	svelte-virtual-list-row {
 		overflow: hidden;
 	}
@@ -163,7 +170,7 @@
 	>
 		{#each visible as row (row.index)}
 			<svelte-virtual-list-row>
-				<slot item={row.data}>Missing template</slot>
+				<slot index={row.index} item={row.data}>Missing template</slot>
 			</svelte-virtual-list-row>
 		{/each}
 	</svelte-virtual-list-contents>