Improve side UI for fullwide.

Props for bugtesting, @jvisick

This adds padding left and right on fullwide columns. We can revisit this in the future.

blocks/library/columns/editor.scss

@@ -8,10 +8,14 @@
 	&:last-child {
 		margin-right: -$block-padding;
 	}
-}
 
+	// This max-width is used to constrain the main editor column, it should not cascade into columns
+	.editor-block-list__block {
+		max-width: none;
+	}
+}
 
-.editor-block-list__block[data-align="full"] .wp-block-columns .editor-block-list__layout,
+// Wide: show no left/right margin on wide, so they stack with the column side UI
 .editor-block-list__block[data-align="wide"] .wp-block-columns .editor-block-list__layout {
 	&:first-child {
 		margin-left: 0;
@@ -20,3 +24,16 @@
 		margin-right: 0;
 	}
 }
+
+// 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
+// @todo this could be revisited, by for example showing this margin only when the parent block was selected first
+// Then at least an unselected columns block would be an accurate preview
+.editor-block-list__block[data-align="full"] .wp-block-columns .editor-block-list__layout {
+	&:first-child {
+		margin-left: $block-side-ui-padding;
+	}
+	&:last-child {
+		margin-right: $block-side-ui-padding;
+	}
+}

editor/components/block-list/style.scss

@@ -242,7 +242,7 @@
 			margin-right: -$block-side-ui-padding;
 		}
 
-		.editor-block-list__block-edit {
+		> .editor-block-list__block-edit {
 			margin-left: -$block-padding;
 			margin-right: -$block-padding;
 
@@ -252,7 +252,7 @@
 			}
 		}
 
-		.editor-block-list__block-edit:before {
+		> .editor-block-list__block-edit:before {
 			left: 0;
 			right: 0;
 			border-left-width: 0;