Document Chrome App window draggability via webkit-app-region CSS.

chrome/common/extensions/docs/templates/intros/app_window.html

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+<h2 id="draggable">A note on draggable window areas</h2><p>
+
+<p>
+Windows can be moved by dragging on the default title bar,
+but frameless windows do not have that affordance.
+The CSS property <code>-webkit-app-region</code> can be given the values
+<code>drag</code> and <code>no-drag</code> to allow content of a window
+to create draggable areas. Care must be taken when setting a high level
+DOM node to drag, any child that is intractable/clickable must be set to
+no-drag. E.g.
+<p>
+
+<pre data-filename="window.html">
+&lt;div class='title-area'>
+  A draggable area. &lt;div class='title-button'>clickable button&lt;/div>
+&lt;/div>
+</pre>
+
+<pre data-filename="style.css">
+.title-area {
+  -webkit-app-region: drag;
+}
+.title-button {
+  -webkit-app-region: no-drag;
+}
+</pre>
+
+<p>
+See the <a href="https://github.com/GoogleChrome/chrome-app-samples/tree/master/samples/frameless-window">Frameless Window Sample</a> as an example.
+</p>
+