Maybe the theme does not provide an action hook for wp_head. This is crucial since the latest version of Lightbox with PhotoSwipe does not use the old template_redirect filter due to compatibility issues with CDN and caching plugins.
Also see here:
https://developer.wordpress.org/reference/hooks/wp_head/
The description says: Although this is theme-dependent, it is one of the most essential theme hooks, so it is widely supported.
So to me it looks like a theme may also not use wp_head() – in this case my plugin would never be called to add the required image attributes :-(.
And the use in my plugin:
https://github.com/arnowelzel/lightbox-photoswipe/blob/master/lightbox-photoswipe.php#L83
To verify this theory:
In the file /wp_content/plugins/lightbox-photoswipe/lightbox-photoswipe.php replace add_action('wp_head', array($this, 'bufferStart'), 2000); with this: add_action('template_redirect', array($this, 'bufferStart'), 2000);.
If this works, then I will add a backend option to select which hook should be used.
I have tried changing mentioned line but this does not seems to help. Playing around I found that settings for „Show WordPress galleries and Gutenberg gallery blocks in separate lightboxes“ actually outputs data-gallery-id (using both wp_head and template_redirect) so the plugin is called.
Sage 9 does use wp_head() https://github.com/roots/sage/blob/master/resources/views/partials/head.blade.php#L5
The gallery ids are not added during output buffer handling but a bit earlier when the gallery shortcode or Gutenberg gallery block is processed. In a second step, Lightbox with PhotoSwipe adds a hook for wp_head which starts a new output buffer with ob_start() to fetch the page before it is send to the browser. So something in the template or another plugin causes a conflict here.
At https://github.com/roots/sage/blob/master/app/filters.php#L49 I see that sage also uses ob_start() – but there is no ob_end_flush() anywhere in the code. Maybe this is the problem. However – I can not solve this without building a test website with the exact same template and same settings as yours. I’ll see what I can do – but this may take a while.
Thank you so much, I will also try to look at it myself – regarding my setup, problem can be replicated on the latest WP (5.3.2) install with unmodified Sage 9 theme without any other plugin installed.
I have the same problem, also been trying to fix with ob_end_flush, no success till now. Hope to fix it, so we can start using this (excellent) plugin. We use sage 9 for many websites.
I’m nearly sure I had it working on a sage theme with the beta in this issue/thread: https://wordpress.org/support/topic/new-version-of-the-plugin-not-working-properly/ But for now I can’t get it to work anymore, maybe it was with an older wp version (just one or 2 minor versions older).
