From f0df53365d86d3002c55ad9709ce5653e5417dde Mon Sep 17 00:00:00 2001
From: Simon <simon.huenecke@gmail.com>
Date: Thu, 10 Sep 2026 18:23:51 +0000
Subject: [PATCH] Split the reels view into panels

A panel can be split to the right or below, and the panel that appears can
be split again, so any arrangement is reachable. The layout is a binary
tree and the leaves, read in order, are the panels of a step.

Scrolling drives all of them: with N panels a step covers N videos and one
swipe advances the whole set. That runs through every index in the feed --
opening on a video, realigning after a rotation, the prefetch buffer, the
render window -- all of which now convert between a video and the step
that holds it.

Each panel has its own sound, so two can play at once if that is what you
want. A new panel inherits the feed-wide setting rather than starting
muted, so a step built later doesn't disagree with what is already on
screen, and the feed-wide button reads as muted only while every panel is.

Two things fall out of panels that are worth knowing. The render window
narrows as panels are added -- five steps ahead of a four-panel split
would be twenty live <video> elements -- so splitting does not multiply
decoding. And splitting rebuilds around the video you are on rather than
keeping it in the panel you split from: steps are aligned to the panel
count, so it stays on screen but not necessarily first.

The per-video helpers were already written against an element holding one
video's controls, so they took a panel unchanged; a slide became the
container that fans out over them.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QBDkEXP4htyXTCZUwMLphd
---
 frontend/css/style.css |  90 ++++++++-
 frontend/js/feed.js    | 433 +++++++++++++++++++++++++++++++----------
 2 files changed, 415 insertions(+), 108 deletions(-)

diff --git a/frontend/css/style.css b/frontend/css/style.css
index d86937c..7ee3091 100644
--- a/frontend/css/style.css
+++ b/frontend/css/style.css
@@ -1324,7 +1324,7 @@ body.theme-light .video-menu-btn {
 }
 
 /* Live streams can't be seeked, so hide the feed scrubber. */
-.feed-slide.is-live .feed-timeline {
+.feed-pane.is-live .feed-timeline {
     display: none;
 }
 
@@ -2158,6 +2158,9 @@ body.feed-mode-open .mode-toggle-btn .icon-svg {
     flex: none;
 }
 
+/* One screenful. It holds the pane tree rather than a video directly: with
+   several panes, a step shows that many videos at once and one swipe advances
+   the whole set. */
 .feed-slide {
     position: relative;
     width: 100%;
@@ -2165,6 +2168,34 @@ body.feed-mode-open .mode-toggle-btn .icon-svg {
     height: 100dvh;
     scroll-snap-align: start;
     scroll-snap-stop: always;
+    overflow: hidden;
+    background: #000;
+}
+
+/* A branch of the tree: two children sharing the space, side by side or
+   stacked. Nesting these is what makes any arrangement reachable. */
+.feed-split {
+    display: flex;
+    width: 100%;
+    height: 100%;
+    gap: 2px;
+}
+
+.feed-split.is-row { flex-direction: row; }
+.feed-split.is-col { flex-direction: column; }
+
+/* min-* is what stops a flex child refusing to shrink below its content. */
+.feed-split > .feed-pane,
+.feed-split > .feed-split {
+    flex: 1 1 0;
+    min-width: 0;
+    min-height: 0;
+}
+
+.feed-pane {
+    position: relative;
+    width: 100%;
+    height: 100%;
     display: flex;
     align-items: center;
     justify-content: center;
@@ -2172,6 +2203,45 @@ body.feed-mode-open .mode-toggle-btn .icon-svg {
     background: #000;
 }
 
+.feed-pane-tools {
+    position: absolute;
+    top: 12px;
+    right: 12px;
+    display: flex;
+    gap: 6px;
+    /* Above the video, below the right-hand controls it sits beside. */
+    z-index: 5;
+}
+
+.feed-pane-btn {
+    width: 34px;
+    height: 34px;
+    border-radius: 50%;
+    border: 1px solid rgba(244, 232, 212, 0.18);
+    background: rgba(0, 0, 0, 0.45);
+    color: var(--text-primary);
+    font-size: 14px;
+    line-height: 1;
+    cursor: pointer;
+    backdrop-filter: blur(6px);
+    transition: background 0.2s ease, border-color 0.2s ease;
+}
+
+.feed-pane-btn:hover {
+    background: rgba(0, 0, 0, 0.7);
+    border-color: var(--border-hover);
+}
+
+.feed-pane-mute.is-muted {
+    border-color: var(--accent);
+}
+
+/* The HUD's idle fade takes the panel tools with it, like every other control. */
+body.feed-hud-idle .feed-pane-tools {
+    opacity: 0;
+    transition: opacity 0.4s ease;
+}
+
 .feed-poster,
 .feed-video {
     position: absolute;
@@ -2185,7 +2255,7 @@ body.feed-mode-open .mode-toggle-btn .icon-svg {
     transition: opacity 0.2s ease;
 }
 
-.feed-slide.is-loaded .feed-poster {
+.feed-pane.is-loaded .feed-poster {
     opacity: 0;
 }
 
@@ -2310,6 +2380,22 @@ body.feed-mode-open .mode-toggle-btn .icon-svg {
 }
 
 /* Per-slide favorite (heart) button on the feed HUD right rail. */
+/* The right-hand controls stack upwards from just above the caption, rather
+   than sitting at fixed distances from the bottom. A pane can be a third of
+   the viewport tall, and viewport-scale offsets put these outside it -- clipped
+   by the pane's own overflow and unreachable. */
+.feed-pane .feed-fav-btn,
+.feed-pane .feed-pip-btn,
+.feed-pane .feed-format-btn {
+    position: absolute;
+    right: 12px;
+    bottom: auto;
+    top: 56px;
+}
+
+.feed-pane .feed-pip-btn { top: 100px; }
+.feed-pane .feed-format-btn { top: 144px; }
+
 .feed-fav-btn {
     position: absolute;
     top: auto;
diff --git a/frontend/js/feed.js b/frontend/js/feed.js
index 799a729..eaf89f9 100644
--- a/frontend/js/feed.js
+++ b/frontend/js/feed.js
@@ -74,8 +74,14 @@ App.feed = App.feed || {};
         return (scroller && scroller.clientHeight) || window.innerHeight || 1;
     };
 
-    const clampIndex = function(index) {
+    // Steps, not videos: with N panes a step covers N videos at once.
+    const stepCount = function() {
         const total = (state.loadedVideos || []).length;
+        return total === 0 ? 0 : Math.ceil(total / paneCount());
+    };
+
+    const clampIndex = function(index) {
+        const total = stepCount();
         if (total === 0) return -1;
         return Math.min(total - 1, Math.max(0, index));
     };
@@ -315,7 +321,7 @@ App.feed = App.feed || {};
         if (!video) return;
         if (slide.classList.contains('is-loaded')) {
             if (autoplay) {
-                video.muted = state.feedMuted;
+                video.muted = slide._muted !== false;
                 const playPromise = video.play();
                 if (playPromise && typeof playPromise.catch === 'function') playPromise.catch(() => {});
             }
@@ -352,7 +358,7 @@ App.feed = App.feed || {};
         const streamUrl = App.videos.buildStreamUrlFromSource(resolved);
         const isHls = App.videos.classifySource(resolved).isHls;
 
-        video.muted = state.feedMuted;
+        video.muted = slide._muted !== false;
         video.preload = 'auto';
         video._tearingDown = false;
         applyResume(video, videoData && videoData.id, resolved.isLive);
@@ -407,23 +413,101 @@ App.feed = App.feed || {};
     // Builds (or returns) the .feed-slide element for loadedVideos[index] and
     // inserts it into the DOM in index order, between the top spacer and the
     // sentinel.
-    const createSlide = function(index) {
-        if (slidesByIndex.has(index)) return slidesByIndex.get(index);
-        const v = (state.loadedVideos || [])[index];
-        if (!v) return null;
-        const scroller = getScroller();
-        if (!scroller) return null;
+    // ------------------------------------------------------------------
+    // Panes
+    //
+    // A step of the feed is one screenful, and it holds one or more panes. The
+    // arrangement is a tree: every pane can be split to the right or below,
+    // which replaces it with a two-child split, so any nesting is reachable.
+    // Leaves, read in order, are the panes of a step.
+    //
+    // Scrolling drives all of them at once: with N panes, step i shows videos
+    // i*N to i*N+N-1, and one swipe advances the whole set.
+    // ------------------------------------------------------------------
+    let paneTree = { type: 'pane' };
 
-        const slide = document.createElement('div');
-        slide.className = v.isLive ? 'feed-slide is-live' : 'feed-slide';
-        slide.dataset.videoId = v.id;
-        slide.dataset.index = String(index);
-        slide._videoData = v;
-        slide._index = index;
+    const paneLeaves = function(node, out) {
+        out = out || [];
+        if (node.type === 'pane') out.push(node);
+        else node.children.forEach((child) => paneLeaves(child, out));
+        return out;
+    };
+
+    // Panes per step, which is the stride between one swipe and the next.
+    const paneCount = function() {
+        return paneLeaves(paneTree).length;
+    };
+
+    App.feed.paneCount = paneCount;
+
+    // Replaces `target` with a split holding it and a new pane.
+    const splitPane = function(target, direction) {
+        const replace = function(node) {
+            if (node === target) {
+                return { type: 'split', dir: direction, children: [target, { type: 'pane' }] };
+            }
+            if (node.type === 'split') {
+                node.children = node.children.map(replace);
+            }
+            return node;
+        };
+        paneTree = replace(paneTree);
+        rebuildLayout();
+    };
+
+    // Drops a pane, collapsing the split that held it so no split is ever left
+    // with a single child.
+    const closePane = function(target) {
+        if (paneCount() <= 1) return;
+        const prune = function(node) {
+            if (node.type !== 'split') return node;
+            const kept = node.children.filter((child) => child !== target).map(prune);
+            return kept.length === 1 ? kept[0] : Object.assign(node, { children: kept });
+        };
+        paneTree = prune(paneTree);
+        rebuildLayout();
+    };
+
+    // The pane count is the stride, so changing it renumbers every step. Rebuild
+    // around whatever video is playing so the reader keeps their place.
+    const rebuildLayout = function() {
+        const keepId = state.feedActiveVideoId;
+        teardownAllSlides();
+        const videos = state.loadedVideos || [];
+        let videoIndex = 0;
+        if (keepId != null) {
+            const found = videos.findIndex((v) => String(v.id) === String(keepId));
+            if (found >= 0) videoIndex = found;
+        }
+        state.feedActiveIndex = -1;
+        const step = Math.floor(videoIndex / paneCount());
+        setActive(step);
+        const scroller = getScroller();
+        if (scroller) scroller.scrollTop = step * slideHeight();
+    };
+
+    const teardownAllSlides = function() {
+        slidesByIndex.forEach((slide) => {
+            teardownSlide(slide);
+            slide.remove();
+        });
+        slidesByIndex.clear();
+    };
+
+    // Everything one video needs on screen. Identical to what a slide used to
+    // hold: the per-video helpers below take any element shaped like this, so a
+    // pane and the old single-video slide are interchangeable to them.
+    const buildPane = function(v, index) {
+        const pane = document.createElement('div');
+        pane.className = v.isLive ? 'feed-pane is-live' : 'feed-pane';
+        pane.dataset.videoId = v.id;
+        pane.dataset.index = String(index);
+        pane._videoData = v;
+        pane._index = index;
         const uploaderText = v.uploader || '';
         const liveBadge = v.isLive ? '<span class="live-badge feed-live-badge">● LIVE</span>' : '';
         const favKey = App.favorites ? App.favorites.getKey(v) : null;
-        slide.innerHTML = `
+        pane.innerHTML = `
                 <img class="feed-poster" alt="" loading="lazy" decoding="async">
                 <video class="feed-video" muted playsinline webkit-playsinline preload="none"></video>
                 ${liveBadge}
@@ -433,6 +517,12 @@ App.feed = App.feed || {};
                 </button>
                 <button class="cp-format-btn feed-format-btn" type="button" aria-label="Quality" hidden>HD</button>
                 <div class="cp-format-menu feed-format-menu" role="menu" hidden></div>
+                <div class="feed-pane-tools">
+                    <button class="feed-pane-btn feed-pane-mute" type="button" aria-label="Mute this panel"></button>
+                    <button class="feed-pane-btn feed-pane-split-right" type="button" title="Add a panel to the right" aria-label="Add a panel to the right">⊞</button>
+                    <button class="feed-pane-btn feed-pane-split-down" type="button" title="Add a panel below" aria-label="Add a panel below">⊟</button>
+                    <button class="feed-pane-btn feed-pane-close" type="button" title="Close this panel" aria-label="Close this panel" hidden>✕</button>
+                </div>
                 <div class="cp-flash feed-flash" aria-hidden="true"></div>
                 <div class="feed-info">
                     <h4 class="feed-title"><span class="feed-title-text">${v.title || ''}</span></h4>
@@ -445,17 +535,17 @@ App.feed = App.feed || {};
                     </div>
                 </div>
             `;
-        const poster = slide.querySelector('.feed-poster');
+        const poster = pane.querySelector('.feed-poster');
         App.videos.attachThumbnail(poster, v.thumb);
-        const slideVideo = slide.querySelector('.feed-video');
-        bindTimeline(slide, slideVideo);
-        bindSharedControls(slide, slideVideo, v);
+        const slideVideo = pane.querySelector('.feed-video');
+        bindTimeline(pane, slideVideo);
+        bindSharedControls(pane, slideVideo, v);
 
         // A media error (bad/expired source, network failure, unsupported codec)
         // means this clip can't play -- drop it from the queue. Errors fired by
         // our own teardown (src cleared) carry the _tearingDown flag and are
         // ignored inside markSlideFailed.
-        slideVideo.addEventListener('error', () => markSlideFailed(slide));
+        slideVideo.addEventListener('error', () => markSlideFailed(pane));
 
         // On video end, either loop (handled by the `loop` flag, so `ended`
         // never fires) or auto-scroll to the next clip. We only advance for the
@@ -463,11 +553,15 @@ App.feed = App.feed || {};
         slideVideo.loop = shouldLoop();
         slideVideo.addEventListener('ended', () => {
             if (shouldLoop()) return;
-            if (!slide.classList.contains('is-active')) return;
-            advanceToNext(slide._index);
+            // Only the active step advances, and only from its first pane, so a
+            // short clip in one panel can't drag the others along with it.
+            const slide = pane.closest('.feed-slide');
+            if (!slide || !slide.classList.contains('is-active')) return;
+            if (slide._panes && slide._panes[0] !== pane) return;
+            advanceToNext(slide._step);
         });
 
-        const favBtn = slide.querySelector('.feed-fav-btn');
+        const favBtn = pane.querySelector('.feed-fav-btn');
         if (favBtn && App.favorites) {
             App.favorites.setButtonState(favBtn, App.favorites.has(v));
             favBtn.addEventListener('click', (event) => {
@@ -476,18 +570,110 @@ App.feed = App.feed || {};
             });
         }
 
-        // Insert before the rendered slide with the next-highest index so DOM
-        // order always matches index order; fall back to the sentinel.
+        // Each panel owns its own sound, so two can play at once if that is
+        // what the reader wants. A new panel inherits whatever the feed-wide
+        // control is set to, so a step built later doesn't disagree with the
+        // ones already on screen -- which does mean splitting while unmuted
+        // gives you two audio tracks.
+        const muteBtn = pane.querySelector('.feed-pane-mute');
+        const syncMute = () => {
+            slideVideo.muted = pane._muted;
+            muteBtn.textContent = pane._muted ? '🔇' : '🔊';
+            muteBtn.setAttribute('aria-label', pane._muted ? 'Unmute this panel' : 'Mute this panel');
+            muteBtn.classList.toggle('is-muted', !!pane._muted);
+        };
+        pane._muted = state.feedMuted !== false;
+        pane._syncMute = syncMute;
+        syncMute();
+        muteBtn.addEventListener('click', (event) => {
+            event.stopPropagation();
+            pane._muted = !pane._muted;
+            syncMute();
+            refreshFeedMuteState();
+        });
+
+        return pane;
+    };
+
+    // The feed-wide button reads as muted only while every panel is, so it
+    // can't claim silence over a panel someone unmuted by hand.
+    const refreshFeedMuteState = function() {
+        let anyAudible = false;
+        slidesByIndex.forEach((slide) => {
+            panesOf(slide).forEach((pane) => { if (!pane._muted) anyAudible = true; });
+        });
+        state.feedMuted = !anyAudible;
+        App.feed.updateMuteButton();
+    };
+
+    // Renders the layout tree into elements, handing each leaf the next video.
+    const renderPaneTree = function(node, videos, cursor) {
+        if (node.type === 'pane') {
+            const v = videos[cursor.next++];
+            if (!v) return null;
+            const pane = buildPane(v, cursor.next - 1);
+            pane._node = node;
+            bindPaneTools(pane, node);
+            return pane;
+        }
+        const split = document.createElement('div');
+        split.className = 'feed-split ' + (node.dir === 'row' ? 'is-row' : 'is-col');
+        node.children.forEach((child) => {
+            const el = renderPaneTree(child, videos, cursor);
+            if (el) split.appendChild(el);
+        });
+        return split.childElementCount ? split : null;
+    };
+
+    const bindPaneTools = function(pane, node) {
+        const closeBtn = pane.querySelector('.feed-pane-close');
+        closeBtn.hidden = paneCount() <= 1;
+        pane.querySelector('.feed-pane-split-right').addEventListener('click', (event) => {
+            event.stopPropagation();
+            splitPane(node, 'row');
+        });
+        pane.querySelector('.feed-pane-split-down').addEventListener('click', (event) => {
+            event.stopPropagation();
+            splitPane(node, 'col');
+        });
+        closeBtn.addEventListener('click', (event) => {
+            event.stopPropagation();
+            closePane(node);
+        });
+    };
+
+    // One screenful: the pane tree, filled with this step's run of videos.
+    const createSlide = function(step) {
+        if (slidesByIndex.has(step)) return slidesByIndex.get(step);
+        const videos = state.loadedVideos || [];
+        const per = paneCount();
+        const first = step * per;
+        if (!videos[first]) return null;
+        const scroller = getScroller();
+        if (!scroller) return null;
+
+        const slide = document.createElement('div');
+        slide.className = 'feed-slide';
+        slide.dataset.step = String(step);
+        slide._step = step;
+        const cursor = { next: first };
+        const tree = renderPaneTree(paneTree, videos, cursor);
+        if (!tree) return null;
+        slide.appendChild(tree);
+        slide._panes = Array.from(slide.querySelectorAll('.feed-pane'));
+
+        // Insert before the rendered slide with the next-highest step so DOM
+        // order always matches step order; fall back to the sentinel.
         let ref = getSentinel();
-        let refIndex = Infinity;
+        let refStep = Infinity;
         slidesByIndex.forEach((el, i) => {
-            if (i > index && i < refIndex) {
-                refIndex = i;
+            if (i > step && i < refStep) {
+                refStep = i;
                 ref = el;
             }
         });
         scroller.insertBefore(slide, ref);
-        slidesByIndex.set(index, slide);
+        slidesByIndex.set(step, slide);
         return slide;
     };
 
@@ -496,14 +682,24 @@ App.feed = App.feed || {};
     // does not remove it from the DOM or from slidesByIndex (callers differ
     // on that: removeSlide always does, reset() removes the whole tree at
     // once).
-    const teardownSlide = function(slide) {
-        destroySlidePlayback(slide);
-        if (Array.isArray(slide._sharedControlCleanups)) {
-            slide._sharedControlCleanups.forEach((fn) => { try { fn(); } catch (err) { /* ignore */ } });
-            slide._sharedControlCleanups = null;
+    // A slide is now a container: everything below that used to act on one
+    // video acts on one pane, and the slide fans out over its panes.
+    const panesOf = function(slide) {
+        return (slide && slide._panes) || [];
+    };
+
+    const teardownPane = function(pane) {
+        destroySlidePlayback(pane);
+        if (Array.isArray(pane._sharedControlCleanups)) {
+            pane._sharedControlCleanups.forEach((fn) => { try { fn(); } catch (err) { /* ignore */ } });
+            pane._sharedControlCleanups = null;
         }
     };
 
+    const teardownSlide = function(slide) {
+        panesOf(slide).forEach(teardownPane);
+    };
+
     const removeSlide = function(index) {
         const slide = slidesByIndex.get(index);
         if (!slide) return;
@@ -512,37 +708,24 @@ App.feed = App.feed || {};
         slidesByIndex.delete(index);
     };
 
-    // Re-keys every rendered slide after `removedIndex` was spliced out of
-    // state.loadedVideos: indices past the hole shift down by one so
-    // slidesByIndex (and each slide's _index) stays aligned with the queue.
-    const reindexAfterRemoval = function(removedIndex) {
-        const entries = [];
-        slidesByIndex.forEach((slide, i) => entries.push([i, slide]));
-        slidesByIndex.clear();
-        entries.forEach(([i, slide]) => {
-            const ni = i > removedIndex ? i - 1 : i;
-            slide._index = ni;
-            slide.dataset.index = String(ni);
-            slidesByIndex.set(ni, slide);
-        });
-    };
-
     // Drops a video that failed to load/resolve from the queue and pulls the
-    // next clip into its place. A failed *preload* neighbour leaves the active
-    // video playing untouched; a failed *active* clip is replaced in-place by
-    // the next one (the broken frame is removed and the next clip slides into
-    // the same scroll position, so playback advances without a visible jump).
+    // next clip into its place.
+    //
+    // Removing a video renumbers everything after it, and with panes a step is
+    // a run of videos rather than one -- so the steps from the hole onwards
+    // have to be rebuilt. The steps *before* it keep their videos, which is
+    // what lets a failed preload neighbour leave the active panes playing
+    // untouched; only a failure at or before what's on screen forces the
+    // active step to be rebuilt with it.
     const removeVideoFromQueue = function(videoId) {
         const videos = state.loadedVideos || [];
         const r = videos.findIndex((v) => String(v.id) === String(videoId));
         if (r < 0) return;
-        const prevActive = state.feedActiveIndex;
 
-        // Drop the failed clip's feed slide element from the DOM, then its JSON
-        // from the queue, then re-key the remaining rendered slides.
-        removeSlide(r);
+        // Which video to stay with afterwards: the one that slides into the
+        // failed clip's place, or the last one if it was at the end.
+        const keep = videos[r + 1] || videos[r - 1] || null;
         videos.splice(r, 1);
-        reindexAfterRemoval(r);
 
         // Remove the failed clip's grid card element from the DOM too (the grid
         // shares the queue) and re-pack the remaining cards.
@@ -555,21 +738,22 @@ App.feed = App.feed || {};
             return;
         }
 
-        // The active slot only moves when the removed clip was the active one
-        // (r === prevActive) or, defensively, sat before it.
-        let newActive = prevActive;
-        if (r < prevActive) newActive -= 1;
-        newActive = clampIndex(newActive);
-
-        state.feedActiveIndex = -1; // force setActive to re-promote the slot
-        setActive(newActive);
-
-        if (r <= prevActive) {
-            // Active clip failed: re-anchor scroll onto the clip that slid into
-            // its slot so the snap container stays pinned to the new active.
-            const scroller = getScroller();
-            if (scroller) scroller.scrollTop = newActive * slideHeight();
+        const per = paneCount();
+        const activeLast = (state.feedActiveIndex + 1) * per - 1;
+        if (state.feedActiveIndex >= 0 && r > activeLast) {
+            // The hole is past everything on screen: drop the steps from it
+            // onwards and let the window rebuild them, leaving the active panes
+            // mid-playback exactly as they are.
+            const fromStep = Math.floor(r / per);
+            const stale = [];
+            slidesByIndex.forEach((slide, i) => { if (i >= fromStep) stale.push(i); });
+            stale.forEach(removeSlide);
+            syncWindow(state.feedActiveIndex);
+            return;
         }
+
+        state.feedActiveVideoId = keep ? keep.id : null;
+        rebuildLayout();
     };
 
     // Flags a slide whose video failed and schedules its removal from the queue.
@@ -577,13 +761,13 @@ App.feed = App.feed || {};
     // syncWindow is mid-iteration over it. Teardown-induced errors (src cleared)
     // are ignored via the video's _tearingDown flag, and we only act while the
     // feed is open so late errors after close are harmless.
-    const markSlideFailed = function(slide) {
-        if (!slide || slide._failed || !state.feedOpen) return;
-        const video = slide.querySelector('.feed-video');
+    const markSlideFailed = function(pane) {
+        if (!pane || pane._failed || !state.feedOpen) return;
+        const video = pane.querySelector('.feed-video');
         if (video && video._tearingDown) return;
-        const id = slideVideoId(slide);
+        const id = slideVideoId(pane);
         if (id == null) return;
-        slide._failed = true;
+        pane._failed = true;
         setTimeout(() => removeVideoFromQueue(id), 0);
     };
 
@@ -591,14 +775,40 @@ App.feed = App.feed || {};
     // that fell outside [active - HISTORY_COUNT, active + RENDER_AHEAD], builds
     // any missing ones inside it, and sizes the top spacer to stand in for the
     // slides above the window.
+    // Which steps are materialised. Steps ahead/behind are counted in steps,
+    // but each one now costs a whole step's worth of <video> elements, so
+    // splitting four ways should not put four times as many in the document:
+    // the window narrows as panes are added.
+    //
+    // The top spacer stands in for the steps below `start`, so anything that
+    // sizes the spacer has to agree with this exactly -- disagreeing by a step
+    // shifts every rendered slide and lands the reader on the wrong videos.
+    const windowBounds = function(activeIndex) {
+        const per = paneCount();
+        return {
+            start: Math.max(0, activeIndex - Math.max(1, Math.round(HISTORY_COUNT / per))),
+            end: Math.min(stepCount() - 1,
+                activeIndex + Math.max(1, Math.round(RENDER_AHEAD / per)))
+        };
+    };
+
     const syncWindow = function(activeIndex) {
-        const total = (state.loadedVideos || []).length;
-        if (total === 0) return;
-        const start = Math.max(0, activeIndex - HISTORY_COUNT);
-        const end = Math.min(total - 1, activeIndex + RENDER_AHEAD);
+        if (stepCount() === 0) return;
+        const { start, end } = windowBounds(activeIndex);
+        const per = paneCount();
+        const videos = state.loadedVideos || [];
 
         slidesByIndex.forEach((slide, i) => {
-            if (i < start || i > end) removeSlide(i);
+            if (i < start || i > end) {
+                removeSlide(i);
+                return;
+            }
+            // A step built before all of its videos had arrived is short a pane
+            // or more. Once the rest land it has to be rebuilt, or the videos
+            // that would have filled it are skipped for good: the next step
+            // starts past them.
+            const have = (slide._panes || []).length;
+            if (have < per && videos[i * per + have]) removeSlide(i);
         });
         for (let i = start; i <= end; i++) {
             if (!slidesByIndex.has(i)) createSlide(i);
@@ -612,7 +822,7 @@ App.feed = App.feed || {};
     // JSON, pull the next page so the buffer stays ahead of the rendered window.
     const prefetchIfNeeded = function(activeIndex) {
         const total = (state.loadedVideos || []).length;
-        const bufferAhead = total - 1 - activeIndex;
+        const bufferAhead = total - 1 - ((activeIndex + 1) * paneCount() - 1);
         if (bufferAhead < PREFETCH_PAGES * (state.perPage || 12)
             && state.hasNextPage && !state.isLoading) {
             // The feed is its own reader: the grid's scroll position says
@@ -628,7 +838,7 @@ App.feed = App.feed || {};
         const clamped = clampIndex(index);
         if (clamped < 0) return;
         state.feedActiveIndex = clamped;
-        const activeVideo = (state.loadedVideos || [])[clamped];
+        const activeVideo = (state.loadedVideos || [])[clamped * paneCount()];
         state.feedActiveVideoId = activeVideo ? activeVideo.id : null;
 
         syncWindow(clamped);
@@ -639,21 +849,26 @@ App.feed = App.feed || {};
 
         const activeSlide = slidesByIndex.get(clamped);
         if (activeSlide) {
-            loadSlideSource(activeSlide, activeSlide._videoData, true);
-            requestAnimationFrame(() => measureFeedTitle(activeSlide));
+            panesOf(activeSlide).forEach((pane) => {
+                loadSlideSource(pane, pane._videoData, true);
+                requestAnimationFrame(() => measureFeedTitle(pane));
+            });
         }
 
         slidesByIndex.forEach((slide, i) => {
             if (i === clamped) return;
-            if (i > clamped && i <= clamped + PRELOAD_COUNT) {
-                loadSlideSource(slide, slide._videoData, false);
-            } else if (i >= clamped - KEEP_BEHIND && i < clamped) {
-                // Recently-watched slides stay loaded but paused so scrolling
-                // back resumes seamlessly from where it was paused.
-                if (slide.classList.contains('is-loaded')) pauseSlide(slide);
-            } else if (slide.classList.contains('is-loaded')) {
-                destroySlidePlayback(slide);
-            }
+            const preloadAhead = Math.max(1, Math.round(PRELOAD_COUNT / paneCount()));
+            panesOf(slide).forEach((pane) => {
+                if (i > clamped && i <= clamped + preloadAhead) {
+                    loadSlideSource(pane, pane._videoData, false);
+                } else if (i >= clamped - KEEP_BEHIND && i < clamped) {
+                    // Recently-watched panes stay loaded but paused so scrolling
+                    // back resumes seamlessly from where it was paused.
+                    if (pane.classList.contains('is-loaded')) pauseSlide(pane);
+                } else if (pane.classList.contains('is-loaded')) {
+                    destroySlidePlayback(pane);
+                }
+            });
         });
 
         prefetchIfNeeded(clamped);
@@ -682,19 +897,18 @@ App.feed = App.feed || {};
     // stale scroll position) so an orientation change always keeps the same
     // video playing/focused rather than snapping to a neighbour.
     const realignToActive = function() {
-        const total = (state.loadedVideos || []).length;
-        if (total === 0) return;
+        if (stepCount() === 0) return;
         let index = state.feedActiveIndex;
         if (state.feedActiveVideoId != null) {
             const found = (state.loadedVideos || [])
                 .findIndex((v) => String(v.id) === String(state.feedActiveVideoId));
-            if (found >= 0) index = found;
+            if (found >= 0) index = Math.floor(found / paneCount());
         }
         index = clampIndex(index);
         if (index < 0) return;
         state.feedActiveIndex = index;
         const h = slideHeight();
-        const start = Math.max(0, index - HISTORY_COUNT);
+        const start = windowBounds(index).start;
         const spacer = getTopSpacer();
         if (spacer) spacer.style.height = `${start * h}px`;
         const scroller = getScroller();
@@ -731,8 +945,10 @@ App.feed = App.feed || {};
     App.feed.applyEndBehavior = function() {
         const loop = shouldLoop();
         slidesByIndex.forEach((slide) => {
-            const video = slide.querySelector('.feed-video');
-            if (video) video.loop = loop;
+            panesOf(slide).forEach((pane) => {
+                const video = pane.querySelector('.feed-video');
+                if (video) video.loop = loop;
+            });
         });
     };
 
@@ -802,7 +1018,7 @@ App.feed = App.feed || {};
         if (startVideoId != null) {
             const found = (state.loadedVideos || [])
                 .findIndex((v) => String(v.id) === String(startVideoId));
-            if (found >= 0) startIndex = found;
+            if (found >= 0) startIndex = Math.floor(found / paneCount());
         }
 
         // Force a fresh activation even if the index happens to match.
@@ -824,7 +1040,7 @@ App.feed = App.feed || {};
             hudIdleTimer = null;
         }
         document.body.classList.remove('feed-hud-idle');
-        slidesByIndex.forEach((slide) => destroySlidePlayback(slide));
+        slidesByIndex.forEach((slide) => panesOf(slide).forEach(destroySlidePlayback));
         container.classList.remove('open');
         container.setAttribute('aria-hidden', 'true');
         document.body.classList.remove('feed-mode-open');
@@ -843,10 +1059,15 @@ App.feed = App.feed || {};
         }
     };
 
+    // The feed-wide control now sets every panel at once; each panel still has
+    // its own switch for when they should differ.
     App.feed.toggleMute = function() {
         state.feedMuted = !state.feedMuted;
-        document.querySelectorAll('.feed-video').forEach((video) => {
-            video.muted = state.feedMuted;
+        slidesByIndex.forEach((slide) => {
+            panesOf(slide).forEach((pane) => {
+                pane._muted = state.feedMuted;
+                if (pane._syncMute) pane._syncMute();
+            });
         });
         App.feed.updateMuteButton();
     };