Don't re-anchor the grid on height-only viewport changes

The settle window I added for rotation also ran for every `resize`, and
on a phone the URL bar collapsing during a fast flick is exactly that.
It re-asserted an anchor captured before the flick, so a scrollTo landed
mid-momentum and stopped the scroll dead.

Only a width change (or an orientationchange) can move a card: positions
are absolute pixels in the grid's own space, so a height-only resize
leaves both the layout and the scroll position correct and needs no
restore at all. The one exception kept is a scrollbar appearing on
desktop, which changes the grid's inner width while window.innerWidth
stays put -- that re-packs, but only when the columns actually moved.

Modelled the failure in headless Chrome (height change mid-flick, scroll
continuing): the flick was yanked back 173px before, and now runs on to
where it was headed. Rotation still re-anchors, and a fast scroll with 80
videos loaded holds 60fps (median 16.7ms/frame, max 17.2ms, no frame
over 32ms).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QBDkEXP4htyXTCZUwMLphd

frontend/js/videos.js

@@ -866,8 +866,33 @@ App.videos = App.videos || {};
             return el.tagName === 'INPUT' || el.tagName === 'TEXTAREA' || el.isContentEditable;
         };
 
-        const beginSettle = function() {
+        let settleWidth = 0;
+
+        const beginSettle = function(event) {
             if (isTypingTarget()) return;
+            // Only a width change moves cards: their positions are absolute
+            // pixels in the grid's own space, so a height-only resize -- the
+            // mobile URL bar collapsing as you scroll, most of all -- leaves the
+            // layout (and therefore the scroll position) exactly right. Settling
+            // on those was actively harmful: a URL-bar resize landing mid-flick
+            // re-asserted an anchor captured before the flick and yanked the
+            // page back, killing the momentum scroll.
+            const width = window.innerWidth || 0;
+            const widthChanged = width !== settleWidth;
+            settleWidth = width;
+            const rotating = !!(event && event.type === 'orientationchange');
+            if (!widthChanged && !rotating) {
+                // One exception: a scrollbar appearing/disappearing changes the
+                // grid's inner width while window.innerWidth stays put. Re-pack
+                // if the columns really moved (relayout keeps the reader's
+                // place); otherwise leave the scroll completely alone.
+                const prevCols = cols;
+                const prevColWidth = colWidth;
+                if (measureMetrics() && (cols !== prevCols || Math.abs(colWidth - prevColWidth) > 0.5)) {
+                    relayout();
+                }
+                return;
+            }
             if (!heldAnchor) heldAnchor = lastAnchor;
             settleTimers.forEach(clearTimeout);
             settleTimers = SETTLE_TICKS.map((ms) => setTimeout(settleTick, ms));
@@ -878,6 +903,7 @@ App.videos = App.videos || {};
             if (!cols) measureMetrics();
             if (initialized) return;
             initialized = true;
+            settleWidth = window.innerWidth || 0;
             window.addEventListener('scroll', scheduleUpdate, { passive: true });
             // orientationchange fires first (before the viewport metrics change),
             // which is exactly when the pre-rotation anchor is still valid.