MD Settings: Keep sliding subpages within card

chrome/browser/resources/settings/settings_page/settings_section.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-styles/shadow.html">
 <link rel="import" href="/animation/animation.html">
 
 <dom-module id="settings-section">
   <template>
     <style>
       :host {
         display: flex;
         flex-direction: column;
@@ skipping 18 matching lines @@
         @apply(--shadow-elevation-2dp);
         background-color: white;
         flex: 1;
         overflow: hidden;
       }
 
       :host(.expanding) #card,
       :host(.collapsing) #card,
       :host(.expanded) #card {
         @apply(--shadow-elevation-4dp);
+        /* A stacking context constrains sliding sub-pages to the card. */
+        z-index: 0;
       }
 
       :host(:not(.expanding):not(.expanded)) #card {
         /* Keep the fading-out neon-animatable inside the card. */
         position: relative;
       }
 
       :host(.expanded) #header {
         display: none;
       }
 
       :host([hidden-by-search]) {
         display: none;
       }
     </style>
     <div id="header">
       <div class="title">{{pageTitle}}</div>
     </div>
     <div id="card">
       <content id="content"></content>
     </div>
   </template>
   <script src="settings_section.js"></script>
 </dom-module>