Fix horizontal scrollbar on full-wide blocks with nesting. (#16085)

This fixes #15192.

This PR fixes two issues:

1. There was an issue, probably rebase related, where the columns block had CSS to prevent horizontal scrollbars when fullwide, but which didn't work anymore. I simply fixed the selector again.
2. The recent merge of the clickthrough PR failed to take into account fullwide blocks with nesting, and caused a horizontal scrollbar due to the overlay extending beyond the canvas.

To test this PR, please verify that full wide alignments work as intended. You can test columns, image, embed, media & text.

Please also verify that blocks with nesting work as intended, both in fullwide and not wide situations.

packages/block-editor/src/components/inner-blocks/style.scss

@@ -1,3 +1,5 @@
+// Add clickable overlay to blocks with nesting.
+// This makes it easy to select all layers of the block.
 .block-editor-inner-blocks.has-overlay {
 	&::after {
 		content: "";
@@ -9,3 +11,9 @@
 		z-index: z-index(".block-editor-inner-blocks.has-overlay::after");
 	}
 }
+
+// On fullwide blocks, don't go beyond the canvas.
+[data-align="full"] > .editor-block-list__block-edit > [data-block] .has-overlay::after {
+	right: 0;
+	left: 0;
+}

packages/block-library/src/columns/editor.scss

@@ -21,7 +21,7 @@
 
 // Fullwide: show margin left/right to ensure there's room for the side UI.
 // This is not a 1:1 preview with the front-end where these margins would presumably be zero.
-.editor-block-list__block[data-align="full"] [data-type="core/columns"][data-align="full"] .wp-block-columns > .editor-inner-blocks {
+[data-type="core/columns"][data-align="full"] .wp-block-columns > .editor-inner-blocks {
 	padding-left: $block-padding;
 	padding-right: $block-padding;