[fix] revert #2819 and add code comment (#2883)

* [fix] revert #2819 and add code comment

* lint

* Update packages/kit/src/runtime/client/renderer.js

Co-authored-by: Bjorn Lu <34116392+bluwy@users.noreply.github.com>

* reverse condition

Co-authored-by: Rich Harris <richard.a.harris@gmail.com>
Co-authored-by: Bjorn Lu <34116392+bluwy@users.noreply.github.com>

.changeset/lucky-phones-march.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+[fix] revert #2819 and add code comment

packages/kit/src/runtime/client/renderer.js

@@ -275,44 +275,50 @@ export class Renderer {
 			this._init(navigation_result);
 		}
 
-		const { hash, scroll, keepfocus } = opts || {};
+		// opts must be passed if we're navigating...
+		if (opts) {
+			const { hash, scroll, keepfocus } = opts;
 
-		if (!keepfocus) {
-			getSelection()?.removeAllRanges();
-			document.body.focus();
-		}
+			if (!keepfocus) {
+				getSelection()?.removeAllRanges();
+				document.body.focus();
+			}
 
-		const old_page_y_offset = Math.round(pageYOffset);
-		const old_max_page_y_offset = document.documentElement.scrollHeight - innerHeight;
-
-		await 0;
-
-		const new_page_y_offset = Math.round(pageYOffset);
-		const new_max_page_y_offset = document.documentElement.scrollHeight - innerHeight;
-
-		// After `await 0`, the `onMount()` function in the component executed.
-		// Check if no scrolling happened on mount.
-		const no_scroll_happened =
-			// In most cases, we can compare whether `pageYOffset` changed between navigation
-			new_page_y_offset === Math.min(old_page_y_offset, new_max_page_y_offset) ||
-			// But if the page is scrolled to/near the bottom, the browser would also scroll
-			// to/near the bottom of the new page on navigation. Since we can't detect when this
-			// behaviour happens, we naively compare by the y offset from the bottom of the page.
-			old_max_page_y_offset - old_page_y_offset === new_max_page_y_offset - new_page_y_offset;
-
-		// If there was no scrolling, we run on our custom scroll handling
-		if (no_scroll_happened) {
-			const deep_linked = hash && document.getElementById(hash.slice(1));
-			if (scroll) {
-				scrollTo(scroll.x, scroll.y);
-			} else if (deep_linked) {
-				// Here we use `scrollIntoView` on the element instead of `scrollTo`
-				// because it natively supports the `scroll-margin` and `scroll-behavior`
-				// CSS properties.
-				deep_linked.scrollIntoView();
-			} else {
-				scrollTo(0, 0);
+			const old_page_y_offset = Math.round(pageYOffset);
+			const old_max_page_y_offset = document.documentElement.scrollHeight - innerHeight;
+
+			await 0;
+
+			const new_page_y_offset = Math.round(pageYOffset);
+			const new_max_page_y_offset = document.documentElement.scrollHeight - innerHeight;
+
+			// After `await 0`, the `onMount()` function in the component executed.
+			// Check if no scrolling happened on mount.
+			const no_scroll_happened =
+				// In most cases, we can compare whether `pageYOffset` changed between navigation
+				new_page_y_offset === Math.min(old_page_y_offset, new_max_page_y_offset) ||
+				// But if the page is scrolled to/near the bottom, the browser would also scroll
+				// to/near the bottom of the new page on navigation. Since we can't detect when this
+				// behaviour happens, we naively compare by the y offset from the bottom of the page.
+				old_max_page_y_offset - old_page_y_offset === new_max_page_y_offset - new_page_y_offset;
+
+			// If there was no scrolling, we run on our custom scroll handling
+			if (no_scroll_happened) {
+				const deep_linked = hash && document.getElementById(hash.slice(1));
+				if (scroll) {
+					scrollTo(scroll.x, scroll.y);
+				} else if (deep_linked) {
+					// Here we use `scrollIntoView` on the element instead of `scrollTo`
+					// because it natively supports the `scroll-margin` and `scroll-behavior`
+					// CSS properties.
+					deep_linked.scrollIntoView();
+				} else {
+					scrollTo(0, 0);
+				}
 			}
+		} else {
+			// ...they will not be supplied if we're simply invalidating
+			await 0;
 		}
 
 		this.loading.promise = null;