Add paired browsing interface

.eslintrc

@@ -33,7 +33,7 @@
     "no-unused-vars": [
       "error",
       {
-        "ignoreRestSiblings": true,
+        "ignoreRestSiblings": true
       }
     ],
     "no-useless-call": "error",
@@ -84,7 +84,7 @@
         ],
         "jest/no-hooks": "off",
         "jest/prefer-expect-assertions": "off",
-        "jest/prefer-inline-snapshots": "off",
+        "jest/prefer-inline-snapshots": "off"
       }
     },
     {
@@ -102,7 +102,7 @@
         ],
         "jest/no-hooks": "off",
         "jest/prefer-expect-assertions": "off",
-        "jest/prefer-inline-snapshots": "off",
+        "jest/prefer-inline-snapshots": "off"
       }
     }
   ]

assets/src/admin/paired-browsing/app.css

@@ -0,0 +1,195 @@
+html,
+body {
+	margin: 0;
+	padding: 0;
+	height: 100%;
+	width: 100%;
+}
+
+body {
+	display: flex;
+	flex-direction: column;
+}
+
+body * {
+	font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Oxygen-Sans, Ubuntu, Cantarell, "Helvetica Neue", sans-serif;
+}
+
+#header {
+	height: 32px;
+}
+
+#header * {
+	color: #fff;
+}
+
+#header ul {
+	display: flex;
+	margin: 0;
+	padding: 0;
+	list-style: none;
+	height: 100%;
+}
+
+#header li {
+	align-items: center;
+	display: flex;
+	line-height: 1.5;
+}
+
+#header .iframe-label {
+	margin: 0 auto;
+	font-weight: 600;
+	width: 50%;
+	justify-content: center;
+}
+
+#header .iframe-label.amp {
+	background-color: #0075c2;
+}
+
+#header .iframe-label.non-amp {
+	background-color: #666;
+}
+
+.iframe-label a {
+	text-decoration: none;
+}
+
+.iframe-label .dashicons-migrate {
+	padding-left: 4px;
+	text-decoration: none;
+	vertical-align: text-top;
+}
+
+.iframe-label a:hover,
+.iframe-label a:focus {
+	text-decoration: underline;
+}
+
+#non-amp,
+#amp {
+	flex: 1 0 auto;
+	align-self: stretch;
+}
+
+#non-amp {
+	border-right: 1px solid #666;
+}
+
+#amp {
+	border-left: 1px solid #0075c2;
+}
+
+.container {
+	display: flex;
+	height: 100%;
+}
+
+iframe {
+	width: 100%;
+	height: 100%;
+	border: 0;
+}
+
+.disconnect-overlay {
+	display: none;
+	position: fixed;
+	width: 50%;
+	height: 100%;
+	overflow-y: auto;
+	background-color: rgba(0, 0, 0, 0.4);
+	text-align: center;
+}
+
+.disconnect-overlay::before {
+	content: " ";
+	display: inline-block;
+	vertical-align: middle;
+	height: 100%;
+}
+
+.disconnect-overlay.disconnected {
+	display: block;
+}
+
+.disconnect-overlay	.dialog {
+	width: 480px;
+	background-color: #fff;
+	border-radius: 5px;
+	position: static;
+	margin: 20px 0;
+	padding: 0 20px;
+	display: inline-block;
+	vertical-align: middle;
+	pointer-events: auto;
+}
+
+.disconnect-overlay.amp {
+	left: 50%;
+}
+
+.disconnect-overlay .dialog .dialog-icon {
+	margin: 20px auto;
+}
+
+.disconnect-overlay .dialog .dialog-icon .dashicons-warning {
+	width: 80px;
+	height: 80px;
+	font-size: 80px;
+	color: #fe7f2d;
+}
+
+.disconnect-overlay .dialog .dialog-text {
+	font-size: 16px;
+	padding: 0 10px;
+	max-width: calc(100% - 20px);
+	overflow-wrap: break-word;
+}
+
+.disconnect-overlay .dialog .dialog-buttons {
+	margin-top: 13px;
+	padding: 13px 16px;
+}
+
+.disconnect-overlay .dialog .dialog-buttons button {
+	margin: 5px;
+	border: none;
+	box-shadow: none;
+	border-radius: 5px;
+	font-weight: 600;
+	font-size: 14px;
+	padding: 10px 24px;
+	cursor: pointer;
+}
+
+.skip-link {
+	position: absolute;
+	left: -9999rem;
+	top: 2.5rem;
+	z-index: 999999999;
+}
+
+.skip-link:focus {
+	display: block;
+	left: 6px;
+	top: 7px;
+	font-size: 14px;
+	font-weight: 600;
+	line-height: normal;
+	padding: 15px 23px 14px;
+	z-index: 100000;
+	right: auto;
+	background-color: #fff;
+	border-radius: 3px;
+	box-shadow: 0 0 2px 2px rgba(0, 0, 0, 0.6);
+	clip: auto;
+	clip-path: none;
+	color: #0075c2;
+	height: auto;
+	width: auto;
+}
+
+.hidden {
+	display: none;
+}