New enabled View modes for bundle have duplicate fields in layout

I could have sworn we had an issue for this already, but I can't find it.

When a view mode is enabled, it copies all the values from the default view mode.
So if LB is enabled for the default, it will be for the new one. And then it also copies over the sections from the default, before going through and initializing all the fields as new components...

#2990517-3: Adding a display mode to a content type using layout, and disabling layout on that new display mode removes the layout_builder__layout field and breaks layout in already configured display modes. was the issue I was remembering. That issue is about what happens when trying to get around this bug, and is way worse (data loss!).
I haven't compared the test from this and that issue, but I bet they are similar.

This sounds major or critical; marking critical pending feedback.

When I follow the summary test process (before patching), I get a WSOD, while other site pages work.

PDOException: PDO::__construct(): php_network_getaddresses: getaddrinfo failed: Name or service not known in [error]
/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php:420
...
Strange.

This isn't critical at all, but if you get frustrated by this and try to start over, you could easily hit #2990517: Adding a display mode to a content type using layout, and disabling layout on that new display mode removes the layout_builder__layout field and breaks layout in already configured display modes. which IS critical. Settling on major for now.
This is postponed on the above.

Actually not postponed on that, not sure what I was thinking.

Had a test depending on the old behavior :)

+++ b/core/modules/layout_builder/tests/src/Functional/LayoutBuilderTest.php
@@ -373,6 +403,7 @@ public function testLayoutBuilderUiFullViewMode() {
+    $this->drupalPostForm("$field_ui_prefix/display/full", ['layout[enabled]' => TRUE], 'Save');

nice we were relying on the side effect

Committed 19513d5 and pushed to 8.7.x. Thanks!

c/p as 7f448d596d for 8.6.x