multiple fixes

packages/base-styles/_z-index.scss

@@ -66,7 +66,7 @@ $z-layers: (
 
 	// Show sidebar above wp-admin navigation bar for mobile viewports:
 	// #wpadminbar { z-index: 99999 }
-	".edit-post-sidebar": 100000,
+	".edit-post-editor-regions__sidebar": 100000,
 	".edit-widgets-sidebar": 100000,
 	".edit-post-layout .edit-post-post-publish-panel": 100001,
 	// For larger views, the wp-admin navbar dropdown should be at top of

packages/edit-post/src/components/editor-regions/style.scss

@@ -8,12 +8,12 @@
 	// On Mobile keep a margin for the header and admin header
 	// as both of these are fixed
 	top: 0;
-	@include break-small() {
+	@include break-medium() {
 		margin-top: 0;
 
 		// On Desktop position the container as fixed to avoid scroll bleed.
 		position: fixed;
-		top: $admin-bar-height-big;
+		top: $admin-bar-height;
 		left: 0;
 		right: 0;
 		bottom: 0;
@@ -22,13 +22,6 @@
 			top: 0;
 		}
 	}
-
-	@include break-medium() {
-		top: $admin-bar-height;
-		.is-fullscreen-mode & {
-			top: 0;
-		}
-	}
 }
 @include editor-left(".edit-post-editor-regions");
 
@@ -37,7 +30,7 @@
 	display: flex;
 
 	// On Mobile the header is fixed to keep HTML as scrollable.
-	@include break-small() {
+	@include break-medium() {
 		overflow: auto;
 	}
 }
@@ -46,29 +39,43 @@
 	flex-grow: 1;
 
 	// On Mobile the header is fixed to keep HTML as scrollable.
-	@include break-small() {
+	@include break-medium() {
 		overflow: auto;
 	}
 }
 
 .edit-post-editor-regions__sidebar {
 	width: auto; // Keep the sidebar width flexible.
 	flex-shrink: 0;
+	position: fixed !important; // Need to override the default relative positionning
+	z-index: z-index(".edit-post-editor-regions__sidebar");
+	top: 0;
+	right: 0;
+	bottom: 0;
+	left: 0;
+
+	&:empty {
+		display: none;
+	}
 
 	// On Mobile the header is fixed to keep HTML as scrollable.
-	@include break-small() {
+	@include break-medium() {
 		overflow: auto;
+		border-left: $border-width solid $light-gray-500;
+		position: relative !important;
+		z-index: initial;
 	}
 }
 
 .edit-post-editor-regions__header {
 	flex-shrink: 0;
 	height: auto;  // Keep the height flexible.
+	border-bottom: $border-width solid $light-gray-500;
 
 	// On Mobile the header is sticky.
 	position: sticky;
 	top: 0;
-	@include break-small() {
+	@include break-medium() {
 		// Cancel the fixed positionning used in mobile.
 		position: initial;
 		overflow: auto;
@@ -79,10 +86,11 @@
 	height: auto;  // Keep the height flexible.
 	flex-shrink: 0;
 	overflow: auto;
+	border-top: $border-width solid $light-gray-500;
 
 	// On Mobile the footer is hidden
 	display: none;
-	@include break-small() {
+	@include break-medium() {
 		display: block;
 	}
 }

packages/edit-post/src/components/header/header-toolbar/style.scss

@@ -26,7 +26,7 @@
 .edit-post-header-toolbar__block-toolbar {
 	// Stack toolbar below Editor Bar.
 	position: absolute;
-	top: $header-height;
+	top: $header-height + 1px;
 	left: 0;
 	right: 0;
 	background: $white;

packages/edit-post/src/components/header/style.scss

@@ -1,6 +1,6 @@
 .edit-post-header {
+	height: $header-height;
 	padding: $grid-size-small 2px;
-	border-bottom: $border-width solid $light-gray-500;
 	background: $white;
 	display: flex;
 	flex-wrap: wrap;

packages/edit-post/src/components/layout/style.scss

@@ -111,7 +111,6 @@
 		height: $footer-height;
 		padding: 0 $grid-size;
 		align-items: center;
-		border-top: $border-width solid $light-gray-500;
 		font-size: $default-font-size;
 	}
 }

packages/edit-post/src/components/sidebar/style.scss

@@ -1,7 +1,4 @@
 .edit-post-sidebar {
-	z-index: z-index(".edit-post-sidebar");
-	width: $sidebar-width;
-	border-left: $border-width solid $light-gray-500;
 	background: $white;
 	color: $dark-gray-500;
 
@@ -12,6 +9,10 @@
 		-webkit-overflow-scrolling: touch;
 	}
 
+	@include break-medium() {
+		width: $sidebar-width;
+	}
+
 	> .components-panel {
 		border-left: none;
 		border-right: none;

packages/edit-post/src/components/text-editor/style.scss

@@ -1,4 +1,5 @@
 .edit-post-text-editor {
+	position: relative;
 	width: 100%;
 
 	// Always show outlines in code editor