Previously: #21868, #21467

This pull request seeks to revert more of the block context implementation introduced in #21467, effectively bypassing the pre_render_block filter introduced in #21467 altogether.

This is being proposed as a combination of:

1. #21467 inadvertently regressed the behavior of render_block on inner blocks. The filter is only being run on the top-level blocks of a post content.
   • https://wordpress.slack.com/archives/C02QB2JS7/p1587958680314500 (link requires registration)
2. Worries about additional overhead in WP_Block class construction
   • #21797 (comment)

The pull request is proposed in a way which minimizes the changes necessary to bypass this filter to restore most of the behavior which existed prior to #21467. In doing so, it leaves some lingering unused code. This is done largely due to time constraints, given that Gutenberg 8.0.0RC is proposed to be published today (Monday April 27). It may be possible that this be refined in time for the final release, though it's not strictly necessary to do so.

Testing Instructions:

Repeat testing instructions from #21467. Optionally, it was made easier to test in later development of #21467, using the "Gutenberg Test Block Context" plugin included in the default development environment, via the associated "Test Context Provider" block.

To verify a fix of the issue reported in Slack:

1. Create a post which includes a nested block, like a Group block with a paragraph in it.
2. Preview the post
3. Add a render_block filter somewhere in your local environment (see patch below)
4. Verify that the filter is called for both the outer and inner blocks (in the patch example, you should see both "core/group" and "core/paragraph", not just "core/group" like you see in the current master).

diff --git a/lib/compat.php b/lib/compat.php
index e38f339030..cc5686d35e 100644
--- a/lib/compat.php
+++ b/lib/compat.php
@@ -177,0 +178,5 @@ function gutenberg_get_post_from_context() {
+
+add_filter( 'render_block', function( $block_content, $block ) {
+	var_export( $block['blockName'] );
+	return $block_content;
+}, 10, 2 );