Track last editor focused and refocus when window refocuses

In desktop VScode, focus gets lost when Alt-Tabbing or switching tab views
(unlike tabbing around a browser, which remembers)

Do some focus-tracking for the various codemirror instances. This is likely not
perfect yet, but it's a lot better

Addresses jpolitz/pyret-parley-vscode#13

Author: jpolitz

src/web/js/beforePyret.js

@@ -152,6 +152,7 @@ $(function() {
       animationDiv = null;
     }
   }
+  let activeEditor = null;
   CPO.makeEditor = function(container, options) {
     var initial = "";
     if (options.hasOwnProperty("initial")) {
@@ -230,6 +231,9 @@ $(function() {
     cmOptions = merge(cmOptions, options.cmOptions || {});
 
     var CM = CodeMirror.fromTextArea(textarea[0], cmOptions);
+    CM.on("focus", () => {
+      activeEditor = CM;
+    });
 
     function firstLineIsNamespace() {
       const firstline = CM.getLine(0);
@@ -1417,6 +1421,10 @@ $(function() {
 
   });
 
+  window.addEventListener("focus", (e) => {
+    if(activeEditor) { activeEditor.focus(); }
+  });
+
   function makeEvent() {
     const handlers = [];
     function on(handler) {