[MD Bookmarks] Add hover border to sidebar splitter.

chrome/browser/resources/md_bookmarks/app.html

Index: chrome/browser/resources/md_bookmarks/app.html
diff --git a/chrome/browser/resources/md_bookmarks/app.html b/chrome/browser/resources/md_bookmarks/app.html
index faea7531ee431868543828852ee69046dfbf2d3b..115c11df1f47f6d051c0a6d6b63dc12c1e577110 100644
--- a/chrome/browser/resources/md_bookmarks/app.html
+++ b/chrome/browser/resources/md_bookmarks/app.html
@@ -30,8 +30,17 @@
       }
 
       #splitter {
+        box-sizing: border-box;
         cursor: e-resize;
         flex: 0 0 var(--splitter-width);
+        opacity: 0;
+      }
+
+      #splitter:hover,
+      #splitter.splitter-active {
+        -webkit-border-start: 1px solid rgba(0, 0, 0, 0.1);
+        opacity: 1;
+        transition: opacity 100ms ease-out;
       }
 
       :host-context([dir='rtl']) #splitter {