Make overlay.windows a set.

common/modules/base.jsm

@@ -378,6 +378,9 @@ function values(obj) iter(function values() {
     else if (isinstance(obj, ["Generator", "Iterator", Iter]))
         for (let k in obj)
             yield k;
+    else if (iter.iteratorProp in obj)
+        for (let v of obj)
+            yield v;
     else
         for (var k in obj)
             if (hasOwnProperty(obj, k))

common/modules/main.jsm

@@ -231,7 +231,7 @@ overlay.overlayWindow(Object.keys(config.overlays),
     },
 
     cleanup: function cleanup(window) {
-        overlay.windows = overlay.windows.filter(w => w != window);
+        overlay.windows.remove(window);
 
         Cu.nukeSandbox(this.jsmodules);
     },
@@ -252,7 +252,7 @@ overlay.overlayWindow(Object.keys(config.overlays),
 
         defineModule.loadLog.push("Loaded in " + (Date.now() - this.startTime) + "ms");
 
-        overlay.windows = array.uniq(overlay.windows.concat(window), true);
+        overlay.windows.add(window);
     },
 
     loadModule: function loadModule(module, prereq, frame) {

common/modules/overlay.jsm

@@ -404,19 +404,22 @@ var Overlay = Module("Overlay", XPCOM([Ci.nsIObserver, Ci.nsISupportsWeakReferen
 
     get activeModules() this.activeWindow && this.activeWindow.dactyl.modules,
 
-    get modules() this.windows.map(w => w.dactyl.modules),
+    get modules() [w.dactyl.modules for (w of this.windows)],
 
     /**
      * The most recently active dactyl window.
      */
-    get activeWindow() this.windows[0],
+    get activeWindow() {
+        let win = this._activeWindow && this._activeWindow.get();
+        return this.windows.has(win) && win;
+    },
 
-    set activeWindow(win) this.windows = [win].concat(this.windows.filter(w => w != win)),
+    set activeWindow(win) this._activeWindow = util.weakReference(win),
 
     /**
      * A list of extant dactyl windows.
      */
-    windows: Class.Memoize(() => [])
+    windows: Class.Memoize(() => RealSet())
 });
 
 endModule();