Re-anchor a moving reel on where it is, not what it was playing

A viewport change re-anchors the reel on the video that is playing,
resolved by id rather than by a scroll position the change has already
invalidated. That is what keeps a rotation on the same video.

Mid-swipe it names the wrong one. setActive runs from onScroll's next
frame, so a resize landing before that frame still reads the video being
swiped *away from* -- and realignToActive writes its position straight to
scrollTop, hauling the reel back to it. A phone slides its chrome on every
scroll, so the resize lands mid-swipe on nearly every swipe: the reel went
where the thumb put it, far enough to show the top of the next video at
the bottom of the screen, and was then written back a beat later.

The input guard in onScroll could not catch this. There is no scroll event
to guard -- the write happens inside the resize.

So while the reader is demonstrably driving, take the reel's own position
as the authority and re-anchor on the step it is nearest at the new
height. It still lands on a snap point, so nothing is left straddling two
screens, but the swipe stands. Only a step's worth of disagreement counts
as theirs: a scroller just re-parented into or out of the
picture-in-picture window reads 0, and following that would land them on
the first video instead of their own.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MPZFnNdHbPGDTqQUNiE4ZN

frontend/js/feed.js

@@ -1413,16 +1413,40 @@ App.feed = App.feed || {};
     // video playing/focused rather than snapping to a neighbour.
     const realignToActive = function() {
         if (stepCount() === 0) return;
+        const h = slideHeight();
+        const scroller = getScroller();
+        // Anchoring by id is what keeps a rotation on the same video, because
+        // the scroll position it would otherwise read has been invalidated by
+        // the very change being answered. But mid-swipe that id names the video
+        // being *left*: setActive runs from onScroll's next frame, so a resize
+        // landing before it still reads the old one -- and writing its position
+        // hauls the reel back to it. A phone slides its chrome on every scroll,
+        // so that happened on nearly every swipe.
+        //
+        // While the reader is demonstrably driving, the reel's own position is
+        // the better authority: re-anchor on the step it is nearest at the new
+        // height. That still puts it on a snap point -- no slide left straddling
+        // two screens -- without undoing the swipe.
         let index = state.feedActiveIndex;
         if (state.feedActiveVideoId != null) {
             const found = (state.loadedVideos || [])
                 .findIndex((v) => String(v.id) === String(state.feedActiveVideoId));
             if (found >= 0) index = Math.floor(found / paneCount());
         }
+        const driving = Date.now() - lastInputAt < INPUT_GRACE_MS;
+        if (driving && scroller && h) {
+            // A swipe moves by a step, so only a step's worth of disagreement
+            // is the reader's doing. Anything further apart is the position
+            // being meaningless rather than ahead -- a scroller that has just
+            // been re-parented into (or out of) the picture-in-picture window
+            // reads 0, and following that would land the reel on the first
+            // video instead of theirs.
+            const atScroll = clampIndex(Math.round(scroller.scrollTop / h));
+            if (atScroll >= 0 && Math.abs(atScroll - index) <= 1) index = atScroll;
+        }
         index = clampIndex(index);
         if (index < 0) return;
         state.feedActiveIndex = index;
-        const h = slideHeight();
         const start = windowBounds(index).start;
         // Only write what actually differs. This runs on every frame of a
         // turn, and a spacer height or a scrollTop written back unchanged
@@ -1431,7 +1455,6 @@ App.feed = App.feed || {};
         const spacer = getTopSpacer();
         const spacerHeight = `${start * h}px`;
         if (spacer && spacer.style.height !== spacerHeight) spacer.style.height = spacerHeight;
-        const scroller = getScroller();
         const top = index * h;
         if (scroller && Math.abs(scroller.scrollTop - top) > 0.5) scroller.scrollTop = top;
         const activeSlide = slidesByIndex.get(index);

tests/smoke_feed_swipe.py

@@ -90,6 +90,22 @@ RESNAP = """(top) => {
     document.getElementById('feed-scroll').scrollTop = top;
 }"""
 
+# A swipe, and the chrome reporting itself before the feed has caught up.
+# setActive runs from onScroll's next frame, so doing both in one block --
+# no frame boundary in between -- leaves feedActiveVideoId still naming the
+# video being left, which is the state a phone resize routinely lands in.
+# onResize then re-anchors synchronously, with no scroll event for the
+# input guard in onScroll to see.
+SWIPE_THEN_RESIZE = """() => {
+    const s = document.getElementById('feed-scroll');
+    const before = App.state.feedActiveIndex;
+    s.dispatchEvent(new WheelEvent('wheel', { deltaY: 120, bubbles: true }));
+    s.scrollTop = s.scrollTop + s.clientHeight;
+    const moved = s.scrollTop;
+    window.dispatchEvent(new Event('resize'));
+    return { before: before, moved: moved, after: s.scrollTop, h: s.clientHeight };
+}"""
+
 
 class Checks:
     def __init__(self):
@@ -187,6 +203,22 @@ def main():
         c.ok("re-anchored onto its own snap point",
              abs(resnapped["scrollTop"] - resnapped["expected"]) <= 1, str(resnapped))
 
+        print("\nthe chrome reporting itself before the feed has caught up")
+        page.wait_for_timeout(900)
+        pre = page.evaluate(WHERE)
+        got = page.evaluate(SWIPE_THEN_RESIZE)
+        page.wait_for_timeout(2200)
+        kept = page.evaluate(WHERE)
+        print(f"    (swiped to {got['moved']}px; the resize left it at {got['after']}px)")
+        c.ok("the resize did not haul the reel back to the video being left",
+             got["after"] >= got["moved"] - 1,
+             f"{got['moved']} -> {got['after']}")
+        c.ok("the swipe still landed on the next step",
+             kept["index"] == pre["index"] + 1,
+             f"step {pre['index']} -> {kept['index']}")
+        c.ok("and on that step's snap point",
+             abs(kept["scrollTop"] - kept["expected"]) <= 1, str(kept))
+
         print("\na little height on a near-square viewport is not a rotation")
         page.set_viewport_size(SQUARE)
         page.wait_for_timeout(1800)
@@ -202,8 +234,8 @@ def main():
         c.ok("and the reel rests on its snap point",
              abs(settled["scrollTop"] - settled["expected"]) <= 1, str(settled))
         c.ok("still on the same video",
-             settled["videoId"] == resnapped["videoId"],
+             settled["videoId"] == kept["videoId"],
-             f"{resnapped['videoId']} -> {settled['videoId']}")
+             f"{kept['videoId']} -> {settled['videoId']}")
 
         c.ok("nothing threw along the way", not crashes, str(crashes[:2]))
         browser.close()