Field Layout should not alter fields placed into regions it doesn't know about

Nice test coverage. Proves the problem and the fix!
1 small thing.

+++ b/core/modules/field_layout/src/FieldLayoutBuilder.php
@@ -69,10 +69,12 @@ public function buildView(array &$build, EntityDisplayWithLayoutInterface $displ
         // Move the field from the top-level of $build into a region-specific
         // section.
-        // @todo Ideally the array structure would remain unchanged, see
-        //   https://www.drupal.org/node/2846393.

Before the patch the comments were altogether in 4 lines. Now they are split by the if statement.

If seems like all 4 comment lines should be inside the if statement. With maybe a new comment as to why there would be regions that FieldLayoutBuilder doesn't know about.

@tim.plunkett thank for the change and the updated comment.

RTBC!

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Ahoy. This explains things.

I tested it with older versions of the layout builder patch and confirmed that it resolves the "totally empty layout" problem. Does this mean we can remove the hook_requirements() disallowing them from being enabled at the same time?

I fixed a comma splice on commit:

diff --git a/core/modules/field_layout/src/FieldLayoutBuilder.php b/core/modules/field_layout/src/FieldLayoutBuilder.php
index e818992936..fdbefd5e8d 100644
--- a/core/modules/field_layout/src/FieldLayoutBuilder.php
+++ b/core/modules/field_layout/src/FieldLayoutBuilder.php
@@ -69,7 +69,7 @@ public function buildView(array &$build, EntityDisplayWithLayoutInterface $displ
       foreach ($fields as $name => $field) {
         // If the region is controlled by the layout, move the field from the
         // top-level of $build into a region-specific section. Custom regions
-        // could be set by other code at run-time, these should be ignored.
+        // could be set by other code at run-time; these should be ignored.
         // @todo Ideally the array structure would remain unchanged, see
         //   https://www.drupal.org/node/2846393.
         if (isset($regions[$field['region']])) {

I also backported it because it's a mostly non-disruptive bugfix that should only affect code that's not in core yet and shouldn't affect anyone actually using field layout.

The comments aren't showing up for it but the commits aren't there. I hope VCS intergration isn't broken again.