DevTools: render element tree hover as a block.

third_party/WebKit/Source/devtools/front_end/elements/elementsTreeOutline.css

 /*
  * Copyright (c) 2014 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 .elements-disclosure {
     width: 100%;
     display: inline-block;
     line-height: normal;
@@ skipping 336 matching lines @@
 .elements-disclosure li.selected .gutter-container:not(.has-decorations) {
     -webkit-mask-image: url(Images/toolbarButtonGlyphs_2x.png);
 }
 } /* media */
 
 /** Guide line */
 li.selected {
     z-index: 0;
 }
 
-li.hovered:not(.always-parent) + ol.children, .elements-tree-outline ol.shadow-root, li.selected:not(.always-parent) + ol.children {
+
+li.hovered:not(.always-parent) + ol.children::before {
+    background-color: rgba(56, 121, 217, 0.1);
+    content: "a";
+    position: absolute;
+    top: -1px;
+    bottom: 0;
+    left: -10000px;
+    right: 0;
+}
+
+.elements-tree-outline ol.shadow-root, li.selected:not(.always-parent) + ol.children {
     margin-left: 5px;
     -webkit-padding-start: 6px;
     border-width: 1px;
     border-left-style: solid;
 }
 
 li.hovered:not(.always-parent) + ol.children:not(.shadow-root) {
     border-color: hsla(0,0%,0%,0.1);
 }
 
 .elements-tree-outline ol.shadow-root {
     border-color: hsla(0,0%,80%,1);
 }
 
 li.selected:not(.always-parent) + ol.children {
     border-color: hsla(216,68%,80%,1) !important;
 }