Layout UI: Editing or (re)moving blocks triggers the "dirty form" flag, but there's no message about it.

[klonos]

Steps to reproduce:

1. Navigate to the layout admin UI for say the default front-end layout (/admin/structure/layouts/manage/default) (but any layout really).
2. Remove a block (the "Powered by Backdrop" block for example or any other block you choose) -> there's no "This form has unsaved changes." message (???).
3. Refresh the page -> there it is!

[klonos]

...actually try removing any number of blocks and there is no message. If you click any "configure" link (after you have removed at least one block first) and hit save on the "configure block" dialog (even if you made no changes), then the message comes up.