Reuse feed overscan to reduce rendering during scroll coasts

1 files changed, 15 insertions, 6 deletions

diff --git a/src/features/messages.js b/src/features/messages.js
index 540f942..d3c8e87 100644
--- a/src/features/messages.js
+++ b/src/features/messages.js
@@ -6595,18 +6595,27 @@ export function messagesFeature(ctx) {
     }
     return changed;
   }
-  // On scroll: when the window should move, repaint with the reader's row
-  // held still. One frame at a time.
+  // On scroll: replenish the buffer before the reader reaches its edge,
+  // holding their row still. Coalesce scroll events into one check per frame.
   let windowRaf = 0;
   function windowScrolled(c) {
     if (!c || !c.win || windowRaf) return;
     windowRaf = requestAnimationFrame(() => {
       windowRaf = 0;
-      measureNow(c);
       const list = c.winList ? c.winList() : null;
-      if (!list) return;
-      const next = listWindow(c, list);
-      if (next.start !== c.win.start || next.end !== c.win.end) holdScroll(render);
+      if (!list || !c.win || (!c.win.start && c.win.end >= list.length)) return;
+      // Keep using the 2.5-screen overscan until only one screen remains.
+      // Recentring it every time an offscreen row crossed the ideal boundary
+      // rebuilt the whole pane repeatedly during a coast. Two edge reads
+      // suffice here; measure all rows only when the window actually moves.
+      const rows = document.querySelectorAll('.notes-feed > .row[data-key]');
+      const first = rows[0], last = rows[rows.length - 1], H = window.innerHeight;
+      if (!first || !last) return;
+      const nearTop = c.win.start > 0 && first.getBoundingClientRect().top > -H;
+      const nearBottom = c.win.end < list.length && last.getBoundingClientRect().bottom < 2 * H;
+      if (!nearTop && !nearBottom) return;
+      measureNow(c);
+      holdScroll(render, c);
     });
   }