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Optimize image loading and fix Dark Mode FOUC #5

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Optimize image loading and fix Dark Mode FOUC #5

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656f18e
⚡ Bolt: Optimize image loading and fix Dark Mode FOUC
google-labs-jules[bot] Mar 1, 2026
c24eb8e
Merge pull request #1 from DealsBeam/bolt-image-fouc-optimization-103…
DealsBeam Mar 1, 2026
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3 changes: 3 additions & 0 deletions .jules/bolt.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
## 2025-05-15 - [Blogger Image Serving & FOUC Prevention]
**Learning:** Blogger's `resizeImage` operator is essential for performance, but often ignored in themes. Serving original images and resizing with CSS is a massive bottleneck for LCP. Also, placing dark mode logic at the end of the `<body>` causes a distinct FOUC that impacts perceived performance.
**Action:** Always use `resizeImage` with `srcset` and `sizes` for Blogger templates. Move critical theme-switching logic to the `<head>` to ensure correct rendering from the first frame.
27 changes: 23 additions & 4 deletions gridsense.xml
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Expand Up @@ -12,6 +12,15 @@
-->
<b:include data='blog' name='all-head-content'/>
<title><data:blog.pageTitle/></title>
<script>
//<![CDATA[
// ⚡ Bolt: Immediate theme application to prevent Flash of Unstyled Content (FOUC)
(function() {
const theme = localStorage.getItem('theme') || (window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light');
document.documentElement.setAttribute('data-theme', theme);
})();
//]]>
</script>
<b:if cond='data:view.isHomepage'>
<script type='application/ld+json'>
{
Expand Down Expand Up @@ -1419,7 +1428,14 @@

<!-- Helper for Responsive Image -->
<b:includable id='responsiveImage' var='info'>
<img expr:src='data:info.image' loading='lazy' style='width: 100%; height: 200px; object-fit: cover;'/>
<!-- ⚡ Bolt: Optimized image serving using Blogger's resizeImage for better LCP and data savings -->
<img decoding='async'
expr:alt='data:post.title'
expr:src='resizeImage(data:info.image, 600)'
expr:srcset='resizeImage(data:info.image, 300) + " 300w, " + resizeImage(data:info.image, 600) + " 600w"'
loading='lazy'
sizes='(max-width: 768px) 100vw, 400px'
style='width: 100%; height: 200px; object-fit: cover;'/>
</b:includable>

</b:widget>
Expand Down Expand Up @@ -1755,6 +1771,10 @@
function setTheme(theme) {
html.setAttribute('data-theme', theme);
localStorage.setItem('theme', theme);
updateIcons(theme);
}

function updateIcons(theme) {
if (theme === 'dark') {
iconMoon.style.display = 'none';
iconSun.style.display = 'block';
Expand All @@ -1764,9 +1784,8 @@
}
}

// Check preference
const savedTheme = localStorage.getItem('theme') || (window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light');
setTheme(savedTheme);
// ⚡ Bolt: Use theme already applied in <head> to avoid re-calculating or flashing
updateIcons(html.getAttribute('data-theme') || 'light');

if(toggleBtn) {
toggleBtn.addEventListener('click', () => {
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