In my theme, I placed a featured image of a page above both the content and sidebar columns, spanning both columns, using
<?php the_post_thumbnail('large'); ?>
where "large" size is defined as 980px. Their width is responsive thanks to css. This worked fine until the WordPress 4.4 upgrade.
After the update, which includes the core responsive images feature (loading different images depending on screen size), those featured images are now only shown as 560px wide on every page, instead of 980px. 560px corresponds to medium size of images, as well as to content_width, defined in my functions (it is the width of the content in the main column, used for oEmbeds).
This is the html source code of such an image:
<img class="attachment-large size-large wp-post-image" sizes="(max-width: 560px) 100vw, 560px" srcset="http://marionhansel.local/wp-content/uploads/latendresse-224x126.jpg 224w, http://marionhansel.local/wp-content/uploads/latendresse-560x315.jpg 560w, http://marionhansel.local/wp-content/uploads/latendresse-980x551.jpg 980w, http://marionhansel.local/wp-content/uploads/latendresse-476x268.jpg 476w" alt="La Tendresse" src="http://marionhansel.local/wp-content/uploads/latendresse-980x551.jpg">
How do I make the featured images appear in their "Large" width size (980px), while keeping the (WordPress) responsiveness feature and keeping the content_width (for oEmbeds)?
I guess it has something to do with
(max-width: 560px) 100vw, 560px in the image source, but how do I edit those values?
For now, I have disabled responsive images (using the Disable Responsive Images plugin), and the images show again in their full width (980px), and they are "responsive" thanks to css, but they don't use the WordPress responsiveness.