Convert PaneContainerElement and PaneElement to JS

src/pane-container-element.js

@@ -0,0 +1,40 @@
+const {CompositeDisposable} = require('event-kit')
+
+class PaneContainerElement extends HTMLElement {
+  createdCallback () {
+    this.subscriptions = new CompositeDisposable()
+    this.classList.add('panes')
+  }
+
+  initialize (model, {views}) {
+    this.model = model
+    this.views = views
+    if (this.views == null) {
+      throw new Error('Must pass a views parameter when initializing PaneContainerElements')
+    }
+    this.subscriptions.add(this.model.observeRoot(this.rootChanged.bind(this)))
+    return this
+  }
+
+  rootChanged (root) {
+    const focusedElement = this.hasFocus() ? document.activeElement : null
+    if (this.firstChild != null) {
+      this.firstChild.remove()
+    }
+    if (root != null) {
+      const view = this.views.getView(root)
+      this.appendChild(view)
+      if (focusedElement != null) {
+        focusedElement.focus()
+      }
+    }
+  }
+
+  hasFocus () {
+    return this === document.activeElement || this.contains(document.activeElement)
+  }
+}
+
+module.exports = document.registerElement('atom-pane-container', {
+  prototype: PaneContainerElement.prototype
+})