The data-drupal-messages-fallback element cannot be removed and causes empty regions to render

For more info: https://www.drupal.org/node/3002643

It doesn't make sense to me to force rendering of a block when it's empty. There also isn't a very good CSS approach off the top of my head for countering this (styling based on whether the block has content or not).

Same here. It seems that this would require applying styling to the messages themselves, which isn't the most practical thing if there are several wrappers for that region.

Does anyone know if there's a way to disable this from code?

Hmm. The system_messages_block plugin itself is what is setting the relevant render element variable.

You'd think setting the #include_fallback content key to FALSE in template_preprocess_block() would be able to kill this empty js messages fallback div, but no, that doesn't work.

If all you want to do is remove the data-drupal-messages-fallback div, then you can do that by adding this preprocess function:

function YOUR_THEME_preprocess_block__system_messages_block(&$variables) {
  $variables['content']['#include_fallback'] = FALSE;
}

However:

1. Doing so prevents your site from benefitting from the JS message API. Core isn't using this new API yet, but it might at some point. There might be contrib modules using it already. Think about whether you really want to make your site incompatible with showing status messages generated by JS code.
2. While the above removes that one div, it doesn't prevent the region from rendering. That's because even without the fallback, status messages (whether empty or not) are rendered with a placeholder (see Drupal\Core\Render\Element\StatusMessages::generatePlaceholder()), so when the region renders, it always sees that placeholder as something that is not empty. While there are ways to override that, doing so would introduce bugs related to caching, which would then require additional workarounds to fix.

Since 8.7.0, empty status messages leave empty DIVS causing regions to render even though they are empty.

Item 2 above has been how Drupal 8 has worked since 8.0. Are you sure you had a pre-8.7 site where the region didn't render at all? Are you able to reproduce that with core alone, or did that require additional contrib/custom code?

Fair enough. I think the only reason I've noticed this new placeholder is that I'm building a minimal-markup theme to see how minimal I can go.

I'm also having this issue. The empty tag doesn't bother and the rendering of the empty region is a problem.

Is there a way of doing something like that to prevent the rendering?

{% if page.messages|render|render|striptags is not empty %}

I tried it and wasn't working....

I'm experiencing this issue in 8.9. The preprocess function describe in #7 didn't work for me. Is there another preprocess I can use?

This worked for me using Drupal 8.8.6

function YOUR_THEME_preprocess_html(&$variables) {
$variables["page"]["content"]["messages"]["#include_fallback"] = false;
}