add forum/components/DiscussionListPane

- Extract this from DiscussionPage

js/src/forum/components/DiscussionListPane.js

@@ -0,0 +1,46 @@
+import DiscussionList from './DiscussionList';
+
+const hotEdge = (e) => {
+  if (e.pageX < 10) app.pane.show();
+};
+
+export default class DiscussionListPane {
+  view() {
+    if (!app.discussions.hasDiscussions()) {
+      return;
+    }
+
+    return <div className="DiscussionPage-list">{!$('.App-navigation').is(':visible') && <DiscussionList state={app.discussions} />}</div>;
+  }
+
+  onupdate(vnode) {
+    const $list = $(vnode.dom);
+
+    // When the mouse enters and leaves the discussions pane, we want to show
+    // and hide the pane respectively. We also create a 10px 'hot edge' on the
+    // left of the screen to activate the pane.
+    const pane = app.pane;
+    $list.hover(pane.show.bind(pane), pane.onmouseleave.bind(pane));
+
+    $(document).on('mousemove', hotEdge);
+
+    // If the discussion we are viewing is listed in the discussion list, then
+    // we will make sure it is visible in the viewport – if it is not we will
+    // scroll the list down to it.
+    const $discussion = $list.find('.DiscussionListItem.active');
+    if ($discussion.length) {
+      const listTop = $list.offset().top;
+      const listBottom = listTop + $list.outerHeight();
+      const discussionTop = $discussion.offset().top;
+      const discussionBottom = discussionTop + $discussion.outerHeight();
+
+      if (discussionTop < listTop || discussionBottom > listBottom) {
+        $list.scrollTop($list.scrollTop() - listTop + discussionTop);
+      }
+    }
+  }
+
+  onremove(vnode) {
+    $(document).off('mousemove', hotEdge);
+  }
+}