Only load Gutenberg Polyfill in Gutenberg pages

[youknowriad]

This PR fixes #6847 by avoiding to load the Gutenberg scripts in the classic editor.
This avoids any TinyMCE conflict.

I also added an e2e test to avoid this kind of regressions in the future.

[gziolo]

It seems like a legit fix, however, I'd prefer to have someone else double check PHP logic.

[youknowriad]

It might be handy to define a is_editor_page function to check whether we're loading a Gutenberg page or not

[ellatrix]

Yes, maybe some logic can be shared with pieces of gutenberg_init? Not sure if anything should be different there. It doesn't have the admin and post checks.

[ellatrix]

(I don't really understand why it has these checks here, and gutenberg_init does not.)

[youknowriad]

gutenberg/gutenberg.php

Lines 153 to 159 in 70e9e4f

	if ( isset( $_GET['classic-editor'] ) ) {
	return false;
	}
	if ( ! gutenberg_can_edit_post( $post ) ) {
	return false;
	}

[ellatrix]

Fixes the issue for me. Would be nice to either share the logic with gutenberg_init or explain why different checks are needed. Would also be nice to explain why this is needed: "Ensure the editor module is loaded before third party plugins."

[youknowriad]

"Ensure the editor module is loaded before third party plugins."

This is needed before the previous APIs defined in wp-blocks are loaded in wp-editor and wp-editor is not added as a dependency to these third party plugins. We have a warning showing on the console to help plugin authors migrate but during that period we need to load wp-editor before everything else to avoid breakage.

[youknowriad]

gutenberg_init or explain why different checks are needed

The checks are not different. This has an extra check to ensure we're on the editor page get_current_screen. The gutenberg_init is only run in the replace_editor hook or in post.php

[youknowriad]

Merging as this fixed the issue and it's a temporary polyfill anyway.