From 36ebb53c0f8c592ef5e9876ddad703c5069d80a0 Mon Sep 17 00:00:00 2001
From: Graham Barber <did:plc:57od6g2ic3e3b3kauctjmo3k>
Date: Thu, 14 May 2026 19:49:28 -0700
Subject: [PATCH] fix(runtime): smooth scroll between slides on navigation
 events
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Phase 2 added scrollToSlide to navigate.next/previous action lists so
remote-driven nav (from the presenter tab) would also scroll the
audience tab. But scrollToSlide was using behavior:"instant" — which
was correct for its original use as the initial state-entry action
(load straight to the URL-hash slide) but wrong for nav transitions,
which we want animated.

Split into two actions:
  - scrollToSlide: behavior:"smooth", used by navigate.next/previous.
  - jumpToSlide: behavior:"instant", used as the entry action on
    initial state so the page loads positioned without animating
    through every preceding slide.

Verified mid-scroll sampling: scrollTop transitions 0 → 33 → 900 over
~800ms, no longer a single-frame jump.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
---
 packages/runtime/actor/machine.ts |  2 +-
 packages/runtime/actor/setup.ts   | 24 ++++++++++++++++++++----
 2 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/packages/runtime/actor/machine.ts b/packages/runtime/actor/machine.ts
index b64c95e..32797a7 100644
--- a/packages/runtime/actor/machine.ts
+++ b/packages/runtime/actor/machine.ts
@@ -26,7 +26,7 @@ export const presentation = machineBase.createMachine({
       },
     },
     independent: {
-      entry: ["scrollToSlide"],
+      entry: ["jumpToSlide"],
       on: {
         "navigate.next": {
           actions: [
diff --git a/packages/runtime/actor/setup.ts b/packages/runtime/actor/setup.ts
index 7779146..a1967c0 100644
--- a/packages/runtime/actor/setup.ts
+++ b/packages/runtime/actor/setup.ts
@@ -26,12 +26,28 @@ export const machineBase = setup({
       }
     }),
     /**
-     * Scroll the audience-mode viewport to the current slide. No-op in
-     * presenter mode (no scrolling there) and on empty decks. Used both
-     * as the entry action for initial state and as a side effect of
-     * remote-driven navigation events broadcast from the presenter tab.
+     * Smooth-scroll the audience-mode viewport to the current slide.
+     * Used as a side effect of navigation events (toolbar buttons,
+     * keyboard, broadcast from the presenter tab) so the audience
+     * sees a clean animation between slides. No-op in presenter mode
+     * (no scrolling there) and on empty decks.
      */
     scrollToSlide({ context }) {
+      if (context.role !== "audience") return;
+      if (context.slides.length === 0) return;
+      const slide = context.slides[context.currentIndex];
+      if (!slide) return;
+      document.querySelector(`#${slide.id}`)?.scrollIntoView({
+        behavior: "smooth",
+      });
+    },
+
+    /**
+     * Instant-jump variant used as the initial state entry action so the
+     * page loads positioned at the URL-hash slide without animating
+     * through every slide above it.
+     */
+    jumpToSlide({ context }) {
       if (context.role !== "audience") return;
       if (context.slides.length === 0) return;
       const slide = context.slides[context.currentIndex];
-- 
2.51.2