Make :yank :extensions work as expected.

common/content/commandline.js

@@ -1215,7 +1215,9 @@ var CommandLine = Module("commandline", {
         function observe(str, highlight, dom) {
             buffer.push(dom && !isString(str) ? util.domToString(dom) : str);
         }
+        this.savingOutput = true;
         dactyl.trapErrors.apply(dactyl, [fn, self].concat(Array.slice(arguments, 2)));
+        this.savingOutput = false;
         return buffer.join("\n");
     }
 }, {

common/content/dactyl.js

@@ -1804,6 +1804,7 @@ var Dactyl = Module("dactyl", XPCOM(Ci.nsISupportsWeakReference, ModuleBase), {
                                         &#xa0;on restart)</>;
                     return <></>;
                 }
+                let waiting = true;
                 AddonManager.getAddonsByTypes(["extension"], function (extensions) {
                     if (args[0])
                         extensions = extensions.filter(function (extension) extension.name.indexOf(args[0]) >= 0);
@@ -1823,7 +1824,10 @@ var Dactyl = Module("dactyl", XPCOM(Ci.nsISupportsWeakReference, ModuleBase), {
                         dactyl.echoerr("Exxx: No extension matching " + filter.quote());
                     else
                         dactyl.echoerr("No extensions installed");
+                    waiting = false;
                 });
+                if (commandline.savingOutput)
+                    util.waitFor(function () !waiting);
             },
             { argCount: "?" });
 

common/modules/completion.jsm

@@ -803,8 +803,8 @@ var CompletionContext = Class("CompletionContext", {
      */
     wait: function wait(interruptable, timeout) {
         let end = Date.now() + timeout;
-        while (this.incomplete && (!timeout || Date.now() > end))
-            util.threadYield(false, interruptable);
+        util.waitFor(function () !this.incomplete || (this.timeout && Date.now() > end),
+                     this);
         return this.incomplete;
     }
 }, {

common/modules/util.jsm

@@ -1507,6 +1507,10 @@ var Util = Module("Util", XPCOM([Ci.nsIObserver, Ci.nsISupportsWeakReference]),
             this.yielders--;
         }
     },
+    waitFor: function waitFor(test, self, interruptable) {
+        while (!test.call(self))
+            this.threadYield(false, interruptable);
+    },
 
     yieldable: function yieldable(func)
         function magic() {