Media pager: the slide meant stays on screen through a resize, from the first moment. The pager's resize handler re-showed pager.cur, which the slide observer sets a frame after the pager opens — so a resize in that first frame (the page going full screen for the pager, as the player's full-screen button does: a taller window, the address bar gone) had nothing to re-show, and the pager was left to the browser's re-snap. Now the slide meant is an anchor: the start slide the moment the pager opens, then each slide arrived at; a resize re-shows it (or the tail, when the reader is waiting there), and an arrival the observer reports at a size other than the one the pager last laid out at is taken as the resize's doing, not the reader's: the anchor goes back instead. Hardening: Adam's full-screen button on a clip 23 posts down showed another post's still, which a headless run of the same path (a user-gesture requestFullscreen on the feed clip) and tools/pager-resize-test.js (the window grows as the pager opens, shrinks and grows again, changes shape) did not reproduce; the recorder on his tab is for the next time.

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01J1Kon8bxJoHRpJdSzshCkp

1 files changed, 19 insertions, 1 deletions

diff --git a/src/features/messages.js b/src/features/messages.js
index ae801cd..55deaa1 100644
--- a/src/features/messages.js
+++ b/src/features/messages.js
@@ -12746,6 +12746,7 @@ export function messagesFeature(ctx) {
         if (pager.fromFeed && video) el.classList.remove('show');
       }
       pager.cur = slide;
+      pager.anchor = slide;
       pager.curAt = Date.now();
       pagerArrows(slide);
       if (pager.fromFeed) { const row = feedNow().notes.find((x) => x.id === slide._rowId); markSeen(row || slide._ev); }
@@ -12759,6 +12760,8 @@ export function messagesFeature(ctx) {
       for (const e of entries) {
         if (!e.isIntersecting || e.intersectionRatio < 0.6 || !pager) continue;
         if (pager.flying) continue; // passing, not arriving: the landing is pagerFly's
+        // a slide the screen's new size put there, not the reader: the anchor goes back
+        if (pager.screenH && scroller.clientHeight !== pager.screenH) { pager.onResize(); continue; }
         pager.arrive(e.target);
       }
     }, { root: scroller, threshold: [0.6] });
@@ -12767,6 +12770,8 @@ export function messagesFeature(ctx) {
     document.documentElement.classList.add('no-scroll');
     const first = pagerSlides().find((s) => s.getAttribute('data-k') === want.start) || pagerSlides()[0];
     if (first) pagerShow(first);
+    pager.anchor = first; // the slide meant to be on screen, from the first moment
+    pager.screenH = scroller.clientHeight;
     // the clip on screen's seek bar, every frame (timeupdate comes a few times a second)
     if (video) {
       const p = pager;
@@ -12774,7 +12779,20 @@ export function messagesFeature(ctx) {
       p.raf = requestAnimationFrame(frame);
     }
     // turned on its side (or back), the slides change height: stay on this one
-    pager.onResize = () => { if (pager && pager.cur) pagerShow(pager.cur); };
+    // The screen changed size — a turn of the phone, or the page going full
+    // screen for the pager (the address bar and tabs gone: a taller window).
+    // The posts are a screen tall each, so a scroll position in pixels
+    // deep in the pager now sits in a different post: put the one meant
+    // back on screen. The slide meant is the anchor, set the moment the
+    // pager opens (cur is set by the observer a frame later, and the full
+    // screen's resize beat it: Adam's full-screen button on a clip 23 posts
+    // down landed him on another post's picture) and on each arrival.
+    pager.onResize = () => {
+      if (!pager) return;
+      pager.screenH = pager.scroller.clientHeight;
+      if (pager.onTail) pager.scroller.scrollTop = pager.tail.offsetTop;
+      else if (pager.anchor) pagerShow(pager.anchor);
+    };
     window.addEventListener('resize', pager.onResize);
     // new posts land in the feed while the pager is open; counts move
     pager.tick = setInterval(() => { if (pager) { growPager(); pagerFeedAhead(); if (el.classList.contains('show')) pagerCounts(pager.cur); } }, 1500);