From a2f9742f8a325262469b13672c1c05cba5bfc49a Mon Sep 17 00:00:00 2001
From: daniel-c-harvey <the.real.daniel.harvey@gmail.com>
Date: Thu, 11 Jun 2026 16:08:55 -0400
Subject: [PATCH] fix(parallax): prime parallax position with pre-Blazor init
 script to kill Server->WASM position pop

---
 DeepDrftPublic/Components/App.razor           |  1 +
 .../Interop/parallax/parallax.ts              |  8 ++
 .../wwwroot/scripts/parallax-init.js          | 90 +++++++++++++++++++
 3 files changed, 99 insertions(+)
 create mode 100644 DeepDrftShared.Client/wwwroot/scripts/parallax-init.js
diff --git a/DeepDrftPublic/Components/App.razor b/DeepDrftPublic/Components/App.razor
index f27747e..64236f8 100644
--- a/DeepDrftPublic/Components/App.razor
+++ b/DeepDrftPublic/Components/App.razor
@@ -20,6 +20,7 @@
 
 <body>
 <Routes @rendermode="InteractiveAuto" />
+<script src="@Assets["_content/DeepDrftShared.Client/scripts/parallax-init.js"]"></script>
 <script src="_framework/blazor.web.js"></script>
 <script src=@Assets["_content/MudBlazor/MudBlazor.min.js"]></script>
 <script type="module">
diff --git a/DeepDrftShared.Client/Interop/parallax/parallax.ts b/DeepDrftShared.Client/Interop/parallax/parallax.ts
index bd40a6f..ef0a61f 100644
--- a/DeepDrftShared.Client/Interop/parallax/parallax.ts
+++ b/DeepDrftShared.Client/Interop/parallax/parallax.ts
@@ -141,6 +141,14 @@ export function register(element: HTMLElement, options: RegisterOptions): string
 
     setupAutoHeight(handle);
 
+    // Prime position synchronously so enhanced navigation and WASM handoff have
+    // zero-frame gap. The init script covers cold page load; this covers nav.
+    // Skip under reduced motion — parallax.ts never drives position then.
+    if (!reducedMotion()) {
+        element.setAttribute('data-parallax-active', '');
+        applyParallax(handle);
+    }
+
     return id;
 }
 
diff --git a/DeepDrftShared.Client/wwwroot/scripts/parallax-init.js b/DeepDrftShared.Client/wwwroot/scripts/parallax-init.js
new file mode 100644
index 0000000..0be26c3
--- /dev/null
+++ b/DeepDrftShared.Client/wwwroot/scripts/parallax-init.js
@@ -0,0 +1,90 @@
+/*
+ * parallax-init — synchronous pre-Blazor primer for ParallaxImage.
+ *
+ * Why this exists (the Server→WASM "position pop"):
+ *   App.razor uses InteractiveAuto. On a cold WASM cache the page renders
+ *   server-side, the Server circuit hydrates and parallax.js register()s
+ *   (setting data-parallax-active, which cancels the CSS view() animation),
+ *   then WASM takes over: Blazor disposes the Server component → DisposeAsync
+ *   → unregister() → data-parallax-active is REMOVED. For the multi-frame gap
+ *   until WASM's new instance re-register()s (its IntersectionObserver fires
+ *   asynchronously), the CSS animation resumes and drives background-position-y
+ *   to its own view()-timeline value, which differs from the JS math — that
+ *   brief reversion is the visible pop.
+ *
+ *   This script runs ONCE, synchronously, after the DOM is parsed but before
+ *   Blazor boots, and sets data-parallax-active on every parallax window. That
+ *   attribute is never tied to a component instance, so it survives the
+ *   Server→WASM handoff — the CSS animation stays cancelled the whole time and
+ *   parallax.js remains the sole writer of background-position-y across both
+ *   render modes. register()/unregister() lifecycle is unchanged; on genuine
+ *   navigation the element leaves the DOM entirely, so a stale attribute is moot.
+ *
+ * Why plain JS, not TypeScript:
+ *   A synchronous pre-Blazor primer must be a classic <script> (no module
+ *   graph, no async import). The RCL's TS pipeline emits ESNext modules, which
+ *   are deferred — too late to beat the handoff. This file is therefore a
+ *   hand-authored static asset, deliberately outside Interop/.
+ *
+ * Math parity with parallax.ts applyParallax():
+ *   speed and direction are recovered from the inline custom properties the
+ *   component already sets (--parallax-from / --parallax-to encode
+ *   ParallaxSpeed and InvertDirection), then the SAME progress→position formula
+ *   is applied. Any divergence here would itself be a pop, so the two must stay
+ *   in lockstep — change one, change the other.
+ */
+(function () {
+    'use strict';
+
+    function clamp(value, min, max) {
+        return Math.max(min, Math.min(max, value));
+    }
+
+    function prime(win) {
+        var styles = getComputedStyle(win);
+        var from = parseFloat(styles.getPropertyValue('--parallax-from')) || 0;
+        var to = parseFloat(styles.getPropertyValue('--parallax-to')) || 0;
+
+        // Component encodes: non-inverted → from:0, to:end; inverted → from:end, to:0.
+        // Recover the original inputs from whichever endpoint carries the magnitude.
+        var invert = from > to;
+        var speed = clamp(Math.max(from, to) / 100, 0, 1);
+
+        var rect = win.getBoundingClientRect();
+        var viewportH = window.innerHeight;
+
+        var progress = invert ? rect.top / viewportH : 1 - rect.top / viewportH;
+        progress = clamp(progress, 0, 1);
+
+        var pos = progress * speed * 100;
+
+        var layers = win.querySelectorAll(':scope > .layer');
+        for (var i = 0; i < layers.length; i++) {
+            layers[i].style.backgroundPositionY = pos + '%';
+        }
+
+        // Cancel the CSS animation and adopt the JS value atomically.
+        win.setAttribute('data-parallax-active', '');
+    }
+
+    function primeAll() {
+        // Match parallax.ts: under reduced motion the module skips its scroll
+        // listener and never sets data-parallax-active, leaving the layer at
+        // --parallax-from (CSS animation is already `none` via media query).
+        // Do the same here so the two paths stay consistent.
+        if (window.matchMedia('(prefers-reduced-motion: reduce)').matches) {
+            return;
+        }
+
+        var windows = document.querySelectorAll('.parallax-window');
+        for (var i = 0; i < windows.length; i++) {
+            prime(windows[i]);
+        }
+    }
+
+    if (document.readyState === 'loading') {
+        document.addEventListener('DOMContentLoaded', primeAll);
+    } else {
+        primeAll();
+    }
+})();
