Make VisualEditor gutter area clickable

Code adapted from VisualEditor's ve.init.mw.DesktopArticleTarget-vector.css.

Bug: T162838
Change-Id: I828b97b060d631d7c86bff308802fb05c3dff781

resources/extensions/VisualEditor.core.less

@@ -10,6 +10,17 @@
 		top: @fixed-header-height + @color-height;
 	}
 
+	// Make the gutter area clickable (T162838)
+	.ve-init-mw-desktopArticleTarget .ve-init-mw-target-surface {
+		margin-left: -@content-padding;
+		margin-right: -@content-padding;
+	}
+	.ve-init-mw-desktopArticleTarget .ve-init-mw-target-surface .ve-ce-documentNode,
+	.ve-init-mw-desktopArticleTarget .ve-init-mw-target-surface .ve-ui-surface-placeholder {
+		padding-left: @content-padding;
+		padding-right: @content-padding;
+	}
+
 	@media (min-width: @desktop-small-floor) and (max-width: @desktop-small-width) {
 		.ve-ui-toolbar {
 			margin-top: -.5em;