App: Drop hacks that preserved docs scroll.

src/app/App.tsx

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-// Copyright (c) 2020-2023 The Pybricks Authors
+// Copyright (c) 2020-2026 The Pybricks Authors
 
 import 'react-splitter-layout/lib/index.css';
 import './app.scss';
@@ -52,76 +52,14 @@ const Docs: React.FunctionComponent = () => {
 
     return (
         <iframe
-            // REVISIT: some of this could be moved to the docs repo
-            // so that it runs earlier to prevent flashing in the UI.
-            // The load event doesn't run until after the page is fully
-            // loaded and there doesn't seem to be a reasonable way to
-            // hook into the iframe to know when it has a new document.
             onLoad={(e) => {
-                // HACK: this mess restores the scroll position when
-                // the documentation iframe visibility is toggled.
-                // The iframe will be automatically scrolled to 0 when
-                // CSS `display: none` is set.
-
                 const target = e.target as HTMLIFrameElement;
                 const contentWindow = target.contentWindow;
                 if (!contentWindow) {
                     console.error('could not get iframe content window');
                     return;
                 }
 
-                // the last "good" scrollY value of the iframe
-                let iframeScroll = 0;
-
-                // This bit monitors the visibility.
-                // https://stackoverflow.com/a/44670818/1976323
-                const observer = new IntersectionObserver(
-                    (entries) => {
-                        entries.forEach((entry) => {
-                            // Restore the scroll position when the iframe is
-                            // shown. Toggling the visibility prevents flashing
-                            // the contents from the top of the page before the
-                            // scroll is done.
-                            if (entry.intersectionRatio > 0) {
-                                contentWindow.scrollTo(0, iframeScroll);
-                                contentWindow.document.documentElement.style.visibility =
-                                    'visible';
-                            } else {
-                                contentWindow.document.documentElement.style.visibility =
-                                    'hidden';
-                            }
-                        });
-                    },
-                    {
-                        root: target.parentElement,
-                    },
-                );
-
-                observer.observe(target);
-
-                // Have to remove he observer, otherwise we end up with
-                // conflicting values when a new page is loaded in the iframe.
-                contentWindow.addEventListener('unload', () => {
-                    observer.unobserve(target);
-                });
-
-                // And this keeps track of the scroll position.
-                contentWindow.addEventListener('scroll', () => {
-                    if (contentWindow.scrollY !== 0) {
-                        // Record the current scroll position. If it is 0, it
-                        // could be that the iframe has been hidden or the user
-                        // scrolled there. So we have to ignore 0. But we don't
-                        // want to be one pixel off if the user really did
-                        // scroll there, so we assume that if the last scroll
-                        // is 1, then the user probably went all the way to 0.
-                        if (contentWindow.scrollY === 1) {
-                            iframeScroll = 0;
-                        } else {
-                            iframeScroll = contentWindow.scrollY;
-                        }
-                    }
-                });
-
                 // Override browser default key bindings in iframe.
                 contentWindow.document.addEventListener('keydown', (e) => {
                     // use Ctrl-D/Cmd-D to toggle docs

src/app/app.scss

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-// Copyright (c) 2020-2022 The Pybricks Authors
+// Copyright (c) 2020-2026 The Pybricks Authors
 
 // Custom styling for the App control.
 
@@ -184,7 +184,14 @@ $dark-splitter-color-hover: color.adjust(
 }
 
 // hide the docs and resize separator
-
+// Use visibility rather than display:none so the iframe keeps its layout and
+// scroll position when the docs panel is toggled.
+// Collapse the secondary pane and splitter to zero width so they don't
+// consume space, while keeping the iframe in the DOM.
 div.pb-hide-docs > :not(.layout-pane-primary) {
-    display: none;
+    visibility: hidden;
+    pointer-events: none;
+    flex-basis: 0 !important;
+    width: 0 !important;
+    min-width: 0 !important;
 }