If you use the classic WP Dashboard, and then you click the “Classic Editor” link on a post that was once edited, or created, using the horrific Block Editor, you will be presented with a dialog notifying you of such and offering you the choice of the Block Editor or the Classic Editor:
This dialog quickly becomes infuriating. Like really infuriating. It is not only displayed the first time you re-open a post for editing in the classic editor, but also each time you make an edit in the classic editor and hit the update button, it will also show up.
To stop this behaviour, you need to strip out all of the custom HTML block markers, that are hidden in the post.
To prevent the pop-up from appearing, first edit the post using the classical editor. You will probably now be in Visual mode, and the Visual tab at the top of the post will be active.
Next you need to click on the Text tab, at the top of the post, in order to enter Text mode and to be able to see the hidden HTML tags.
Then remove any occurrence of a start block tag:
<!-- wp:paragraph -->
and a end block tag:
<!-- /wp:paragraph -->
Triple click to select the whole line and then delete.
The above HTML comment block tag is for a paragraph. There will probably be HTML comment block tags for other types of blocks. Therefore, all other HTML comments of the same type, such as
image, and any other HTML comment of that format, also need to be removed. That is to say, any HTML comment tag in this format:
<!-- *wp:******* -->
NOTE: Don’t remove any HTML comments that aren’t of the above format, such as
After you have removed all of these block tag HTML comments and saved/updated the post, the annoying dialog should no longer appear, when you click on the “Classical Editor” link, to edit the post.
When you save/update the first time after making these changes, you may get a yellow notification in the dashboard saying that this version of the post is different from previous block editor edits and that you can restore a previous revision, or something to that effect. Ignore this. You can just dismiss that notification, clicking on the X close button for that notification, and it will not be seen again.