Work around broken Alt keyup in Firefox

Firefox no longer sends keyup events properly for the Alt keys. Try
to sniff out the state of the Alt key by monitoring other events that
include its state.

core/input/keyboard.js

@@ -27,7 +27,8 @@ export default function Keyboard(target) {
         'keyup': this._handleKeyUp.bind(this),
         'keydown': this._handleKeyDown.bind(this),
         'keypress': this._handleKeyPress.bind(this),
-        'blur': this._allKeysUp.bind(this)
+        'blur': this._allKeysUp.bind(this),
+        'checkalt': this._checkAlt.bind(this),
     };
 };
 
@@ -281,6 +282,26 @@ Keyboard.prototype = {
         Log.Debug("<< Keyboard.allKeysUp");
     },
 
+    // Firefox Alt workaround, see below
+    _checkAlt: function (e) {
+        if (e.altKey) {
+            return;
+        }
+
+        let target = this._target;
+        let downList = this._keyDownList;
+        ['AltLeft', 'AltRight'].forEach(function (code) {
+            if (!(code in downList)) {
+                return;
+            }
+
+            let event = new KeyboardEvent('keyup',
+                                          { key: downList[code],
+                                            code: code });
+            target.dispatchEvent(event);
+        });
+    },
+
     // ===== PUBLIC METHODS =====
 
     grab: function () {
@@ -294,6 +315,20 @@ Keyboard.prototype = {
         // Release (key up) if window loses focus
         window.addEventListener('blur', this._eventHandlers.blur);
 
+        // Firefox has broken handling of Alt, so we need to poll as
+        // best we can for releases (still doesn't prevent the menu
+        // from popping up though as we can't call preventDefault())
+        if (browser.isWindows() && browser.isFirefox()) {
+            let handler = this._eventHandlers.checkalt;
+            ['mousedown', 'mouseup', 'mousemove', 'wheel',
+             'touchstart', 'touchend', 'touchmove',
+             'keydown', 'keyup'].forEach(function (type) {
+                document.addEventListener(type, handler,
+                                          { capture: true,
+                                            passive: true });
+            });
+        }
+
         //Log.Debug("<< Keyboard.grab");
     },
 
@@ -301,6 +336,15 @@ Keyboard.prototype = {
         //Log.Debug(">> Keyboard.ungrab");
         var c = this._target;
 
+        if (browser.isWindows() && browser.isFirefox()) {
+            let handler = this._eventHandlers.checkalt;
+            ['mousedown', 'mouseup', 'mousemove', 'wheel',
+             'touchstart', 'touchend', 'touchmove',
+             'keydown', 'keyup'].forEach(function (type) {
+                document.removeEventListener(type, handler);
+            });
+        }
+
         c.removeEventListener('keydown', this._eventHandlers.keydown);
         c.removeEventListener('keyup', this._eventHandlers.keyup);
         c.removeEventListener('keypress', this._eventHandlers.keypress);

core/util/browser.js

@@ -56,6 +56,10 @@ export function isEdge() {
     return navigator && !!(/edge/i).exec(navigator.userAgent);
 }
 
+export function isFirefox() {
+    return navigator && !!(/firefox/i).exec(navigator.userAgent);
+}
+
 export function isWindows() {
     return navigator && !!(/win/i).exec(navigator.platform);
 }