Keep scroll position if we scroll away from the bottom of the term

aka, scroll locking. Allows you to scroll back through logs without new
log data dragging the scroll back down to the next entries as they come
in.

client/app/scripts/components/terminal.js

@@ -142,7 +142,12 @@ class Terminal extends React.Component {
     socket.onmessage = (event) => {
       log('pipe data', event.data.size);
       const input = ab2str(event.data);
+      const scrolledToBottom = term.ydisp === term.ybase;
+      const savedScrollPosition = term.ydisp;
       term.write(input);
+      if (!scrolledToBottom) {
+        this.scrollTo(savedScrollPosition);
+      }
     };
 
     this.socket = socket;
@@ -154,6 +159,18 @@ class Terminal extends React.Component {
     }
   }
 
+  scrollToBottom() {
+    this.scrollTo(this.term.ybase);
+  }
+
+  scrollTo(y) {
+    if (!this.term) {
+      return;
+    }
+    this.term.ydisp = y;
+    this.term.refresh(0, this.term.rows - 1);
+  }
+
   componentDidMount() {
     this._isMounted = true;
     if (this.props.connect) {
@@ -173,6 +190,7 @@ class Terminal extends React.Component {
     const innerNode = ReactDOM.findDOMNode(component.innerFlex);
     this.term.open(innerNode);
     this.term.on('data', (data) => {
+      this.scrollToBottom();
       if (this.socket) {
         this.socket.send(data);
       }