Polish side UI and multi select in nested.

blocks/library/columns/editor.scss

@@ -0,0 +1,11 @@
+// These margins make sure that nested blocks stack/overlay with the parent block chrome
+// This is sort of an experiment at making sure the editor looks as much like the end result as possible
+// Potentially the rules here can apply to all nested blocks and enable stacking, in which case it should be moved elsewhere
+.wp-block-columns .editor-block-list__layout {
+	&:first-child {
+		margin-left: -$block-padding;
+	}
+	&:last-child {
+		margin-right: -$block-padding;
+	}
+}

blocks/library/paragraph/editor.scss

@@ -1,3 +1,8 @@
 .editor-block-list__block:not( .is-multi-selected ) .wp-block-paragraph {
 	background: white;
 }
+
+// Don't show white background when a nesting parent is selected
+.editor-block-list__layout .editor-block-list__layout .editor-block-list__block .wp-block-paragraph {
+	background: inherit;
+}

editor/components/block-list/style.scss

@@ -337,6 +337,7 @@
 		padding: 0;
 		width: $block-side-ui-width;
 		height: 100%; // increase hoverable area
+		//margin-top: -1px; // stack borders on nested side UI
 	}
 
 	// Elevate when selected or hovered

editor/components/block-mover/style.scss

@@ -21,6 +21,7 @@
 		background: $white;
 		border-color: $light-gray-500;
 		border-style: solid;
+		margin-top: -1px; // stack top border
 
 		&:first-child {
 			border-width: 1px 1px 0 1px;

editor/components/block-settings-menu/style.scss

@@ -11,6 +11,7 @@
 		border-color: $light-gray-500;
 		border-style: solid;
 		border-width: 1px;
+		margin-top: -1px; // stack top border
 	}
 
 	.dashicon {