diff --git a/guacamole-common-js/src/main/webapp/modules/Keyboard.js b/guacamole-common-js/src/main/webapp/modules/Keyboard.js
index 43d846502..da45bf72f 100644
--- a/guacamole-common-js/src/main/webapp/modules/Keyboard.js
+++ b/guacamole-common-js/src/main/webapp/modules/Keyboard.js
@@ -55,6 +55,36 @@ Guacamole.Keyboard = function(element) {
      */
     this.onkeyup = null;
 
+    /**
+     * Set of known platform-specific or browser-specific quirks which must be
+     * accounted for to properly interpret key events, even if the only way to
+     * reliably detect that quirk is to platform/browser-sniff.
+     *
+     * @private
+     * @type {Object.<String, Boolean>}
+     */
+    var quirks = {
+
+        /**
+         * Whether the Alt key is actually a modifier for typable keys and is
+         * thus never used for keyboard shortcuts.
+         *
+         * @type {Boolean}
+         */
+        altIsTypableOnly: false
+
+    };
+
+    // Set quirk flags depending on platform/browser, if such information is
+    // available
+    if (navigator && navigator.platform) {
+
+        // The Alt key on Mac is never used for keyboard shortcuts
+        if (navigator.platform.match(/^mac/i))
+            quirks.altIsTypableOnly = true;
+
+    }
+
     /**
      * A key event having a corresponding timestamp. This event is non-specific.
      * Its subclasses should be used instead when recording specific key
@@ -175,6 +205,14 @@ Guacamole.Keyboard = function(element) {
         this.keysym =  keysym_from_key_identifier(key, location)
                     || keysym_from_keycode(keyCode, location);
 
+        /**
+         * Whether the keyup following this keydown event is known to be
+         * reliable. If false, we cannot rely on the keyup event to occur.
+         *
+         * @type {Boolean}
+         */
+        this.keyupReliable = true;
+
         // DOM3 and keyCode are reliable sources if the corresponding key is
         // not a printable key
         if (this.keysym && !isPrintable(this.keysym))
@@ -184,9 +222,13 @@ Guacamole.Keyboard = function(element) {
         if (!this.keysym && key_identifier_sane(keyCode, keyIdentifier))
             this.keysym = keysym_from_key_identifier(keyIdentifier, location, guac_keyboard.modifiers.shift);
 
+        // If a key is pressed while meta is held down, the keyup will
+        // never be sent in Chrome (bug #108404)
+        if (guac_keyboard.modifiers.meta && this.keysym !== 0xFFE7 && this.keysym !== 0xFFE8)
+            this.keyupReliable = false;
+
         // Determine whether default action for Alt+combinations must be prevented
-        var prevent_alt =  !guac_keyboard.modifiers.ctrl
-                        && !(navigator && navigator.platform && navigator.platform.match(/^mac/i));
+        var prevent_alt = !guac_keyboard.modifiers.ctrl && !quirks.altIsTypableOnly;
 
         // Determine whether default action for Ctrl+combinations must be prevented
         var prevent_ctrl = !guac_keyboard.modifiers.alt;
@@ -961,9 +1003,9 @@ Guacamole.Keyboard = function(element) {
                     var defaultPrevented = !guac_keyboard.press(keysym);
                     recentKeysym[first.keyCode] = keysym;
 
-                    // If a key is pressed while meta is held down, the keyup will
-                    // never be sent in Chrome, so send it now. (bug #108404)
-                    if (guac_keyboard.modifiers.meta && keysym !== 0xFFE7 && keysym !== 0xFFE8)
+                    // Release the key now if we cannot rely on the associated
+                    // keyup event
+                    if (!first.keyupReliable)
                         guac_keyboard.release(keysym);
 
                     // Record whether default was prevented
@@ -998,7 +1040,8 @@ Guacamole.Keyboard = function(element) {
 
         } // end if keyup
 
-        // Ignore any other type of event (keypress by itself is invalid)
+        // Ignore any other type of event (keypress by itself is invalid, and
+        // unreliable keyup events should simply be dumped)
         else
             return eventLog.shift();