preserve scroll position when entering page editor

assets/layout.html

@@ -11,6 +11,7 @@
         <script src="/_/modal.js"></script>
         <script src="/_/global-shortcuts.js"></script>
         <script src="/_/history-nav.js"></script>
+        <script src="/_/scroll-sync.js" defer></script>
         <script src="/_/search-suggest.js" defer></script>
         <script src="/_/tree-picker.js"></script>
         <script src="/_/companion.js" defer></script>

assets/scroll-sync.js

@@ -0,0 +1,61 @@
+// Roughly preserve reading position when entering the editor.
+//
+// While reading a page we record how far the center column is scrolled (as a
+// fraction of its scrollable height, keyed by path). When the same path opens
+// in the editor we restore that fraction, so you land near where you were
+// reading instead of at the top. It is fraction-based — the rendered page and
+// the raw-markdown editor have different heights — so the match is deliberately
+// approximate. This only moves the viewport; the caret default (end of doc) is
+// unchanged, so a click still decides where you actually edit.
+(function () {
+    var center = document.querySelector('.center');
+    if (!center) return;
+
+    var key = 'scrollsync:' + location.pathname;
+    // Only restore a freshly-recorded position: a stale entry from earlier in
+    // the tab session would jump a direct ?edit open to the wrong place.
+    var MAX_AGE_MS = 5 * 60 * 1000;
+    var params = new URLSearchParams(location.search);
+    var inEditor = params.has('edit');
+    // Section / insert edits load only a slice of the document into the editor,
+    // so a whole-page reading fraction does not map onto them — restore only for
+    // a full-page ?edit.
+    var fullPageEdit = inEditor && !params.has('section') && !params.has('insert_before');
+
+    function maxScroll() { return Math.max(0, center.scrollHeight - center.clientHeight); }
+
+    if (inEditor) {
+        if (!fullPageEdit) return;
+        var raw = sessionStorage.getItem(key);
+        if (!raw) return;
+        var data;
+        try { data = JSON.parse(raw); } catch (e) { return; }
+        if (!data || typeof data.f !== 'number' || Date.now() - (data.t || 0) > MAX_AGE_MS) return;
+        // Wait for CodeMirror to mount and the column to reach full height, then
+        // map the fraction onto it. A double rAF after load lets CM's initial
+        // layout and focus scroll settle so ours wins.
+        window.addEventListener('load', function () {
+            requestAnimationFrame(function () {
+                requestAnimationFrame(function () {
+                    center.scrollTop = data.f * maxScroll();
+                });
+            });
+        });
+        return;
+    }
+
+    // Reading mode: keep the latest scroll fraction recorded (throttled to one
+    // write per frame) so it is ready the moment the editor opens.
+    var pending = false;
+    center.addEventListener('scroll', function () {
+        if (pending) return;
+        pending = true;
+        requestAnimationFrame(function () {
+            pending = false;
+            var m = maxScroll();
+            try {
+                sessionStorage.setItem(key, JSON.stringify({ f: m > 0 ? center.scrollTop / m : 0, t: Date.now() }));
+            } catch (e) {}
+        });
+    }, { passive: true });
+})();