Improve modal sidebar on mobile.

This fixes scroll bleed when post settings is open on mobile. It also fixes the vertical position of it.

The downside here is that you lose your place in the editor when you open modal. This is similar behavior to WordPress.com, and it's an improvement on the old editor where you'd have to scroll to the metaboxes below the editor regardless. But it might be worth looking into fixing this with JS down the road.

editor/sidebar/style.scss

@@ -1,5 +1,5 @@
 .editor-sidebar {
-	position: fixed;
+	position: sticky;
 	z-index: z-index( '.editor-sidebar' );
 	right: 0;
 	top: $admin-bar-height-big + $header-height;
@@ -8,6 +8,10 @@
 	border-left: 1px solid $light-gray-500;
 	background: $light-gray-300;
 
+	@include break-small() {
+		position: fixed;
+	}
+
 	@include break-medium() {
 		top: $admin-bar-height + $header-height;
 	}
@@ -28,7 +32,27 @@
 
 /* Visual and Text editor both */
 .editor-layout.is-sidebar-opened .editor-layout__content {
-	margin-right: $sidebar-width;
+	margin-left: -200%;
+    float: left;
+	height: 0;
+
+	@include break-small() {
+		margin-left: auto;
+		float: none;
+		height: auto;
+	}
+
+	@include break-medium() {
+		margin-right: $sidebar-width;
+	}
+}
+
+.editor-layout.is-sidebar-opened {
+	overflow: hidden;
+
+	@include break-small() {
+		overflow: auto;
+	}
 }
 
 .editor-layout.is-sidebar-opened .editor-sidebar {