Fake key releases for some Japanese IM keys

Windows behaves very oddly for some Japanese IM keys in that it won't
send a key release event when the key is released. In some keys it never
sends the event, and in some cases it sends the release as the key is
pressed the subsequent time.

core/input/keyboard.js

@@ -164,6 +164,20 @@ export default class Keyboard {
             return;
         }
 
+        // Windows doesn't send proper key releases for a bunch of
+        // Japanese IM keys so we have to fake the release right away
+        const jpBadKeys = [ KeyTable.XK_Zenkaku_Hankaku,
+                            KeyTable.XK_Eisu_toggle,
+                            KeyTable.XK_Katakana,
+                            KeyTable.XK_Hiragana,
+                            KeyTable.XK_Romaji ];
+        if (browser.isWindows() && jpBadKeys.includes(keysym)) {
+            this._sendKeyEvent(keysym, code, true);
+            this._sendKeyEvent(keysym, code, false);
+            stopEvent(e);
+            return;
+        }
+
         stopEvent(e);
 
         // Possible start of AltGr sequence? (see above)

tests/test.keyboard.js

@@ -268,6 +268,47 @@ describe('Key Event Handling', function () {
         });
     });
 
+    describe('Japanese IM keys on Windows', function () {
+        let origNavigator;
+        beforeEach(function () {
+            // window.navigator is a protected read-only property in many
+            // environments, so we need to redefine it whilst running these
+            // tests.
+            origNavigator = Object.getOwnPropertyDescriptor(window, "navigator");
+
+            Object.defineProperty(window, "navigator", {value: {}});
+            if (window.navigator.platform !== undefined) {
+                // Object.defineProperty() doesn't work properly in old
+                // versions of Chrome
+                this.skip();
+            }
+
+            window.navigator.platform = "Windows";
+        });
+
+        afterEach(function () {
+            if (origNavigator !== undefined) {
+                Object.defineProperty(window, "navigator", origNavigator);
+            }
+        });
+
+        const keys = { 'Zenkaku': 0xff2a, 'Hankaku': 0xff2a,
+                       'Alphanumeric': 0xff30, 'Katakana': 0xff26,
+                       'Hiragana': 0xff25, 'Romaji': 0xff24,
+                       'KanaMode': 0xff24 };
+        for (let [key, keysym] of Object.entries(keys)) {
+            it(`should fake key release for ${key} on Windows`, function () {
+                let kbd = new Keyboard(document);
+                kbd.onkeyevent = sinon.spy();
+                kbd._handleKeyDown(keyevent('keydown', {code: 'FakeIM', key: key}));
+
+                expect(kbd.onkeyevent).to.have.been.calledTwice;
+                expect(kbd.onkeyevent.firstCall).to.have.been.calledWith(keysym, "FakeIM", true);
+                expect(kbd.onkeyevent.secondCall).to.have.been.calledWith(keysym, "FakeIM", false);
+            });
+        }
+    });
+
     describe('Escape AltGraph on Windows', function () {
         let origNavigator;
         beforeEach(function () {