Restart wheel coast timing and respond immediately on reversal

1 files changed, 12 insertions, 3 deletions

diff --git a/src/app.js b/src/app.js
index 5d7d22b..52adce4 100644
--- a/src/app.js
+++ b/src/app.js
@@ -6428,9 +6428,18 @@ if (INTENT_URI) {
     if (e.timeStamp < smoothUntil || matchMedia('(pointer: coarse)').matches || !inScope(e.target)) return;
     e.preventDefault();
     const px = e.deltaMode === 1 ? e.deltaY * 40 : e.deltaY;
-    // reverse turns brake first; same-way turns add to the coast already going
-    const keep = Math.sign(v) === Math.sign(px) ? v : 0;
-    coast(keep + (px * 3) / TAU);
+    // A reversal catches the old coast immediately. Discard its fractional
+    // step and frame clock too: after a busy frame, applying the NEW velocity
+    // to the OLD elapsed time made the first reverse step jump. Give the
+    // first notch a small response now, then coast the remaining distance.
+    if (v && px && Math.sign(v) !== Math.sign(px)) {
+      stop();
+      const first = Math.sign(px) * Math.min(16, Math.abs(px * 3 / TAU) * 16);
+      window.scrollBy({ top: first, behavior: 'instant' });
+      coast((px * 3 - first) / TAU);
+    } else {
+      coast(v + (px * 3) / TAU);
+    }
   }, { passive: false });
   // anything else that moves the page takes over from a coast
   window.addEventListener('keydown', (e) => { if (/^(Arrow|Page|Home|End| )/.test(e.key)) stop(); });