keydown_util: Ignore alt-arrow and similar things.

We ignore keystrokes like alt-left-arrow and alt-right-arrow,
so that the browser can do back/forward.

We may need to refine the handling of ctrl/alt/shift in the
future, but now we only support single-key operations.

frontend_tests/node_tests/keydown_util.js

@@ -3,24 +3,35 @@ set_global('$', global.make_zjquery());
 zrequire('keydown_util');
 
 run_test('test_early_returns', () => {
-    var stub = $.create('stub');
-    var opts = {
+    const stub = $.create('stub');
+    const opts = {
         elem: stub,
-        handlers: {},
+        handlers: {
+            left_arrow: () => {
+                throw Error('do not dispatch this with alt key');
+            },
+        },
     };
 
     keydown_util.handle(opts);
-    var keydown_f = stub.keydown;
+    const keydown_f = stub.keydown;
 
-    var e1 = {
+    const e1 = {
         which: 17, // not in keys
     };
 
     keydown_f(e1);
 
-    var e2 = {
+    const e2 = {
         which: 13, // no handler
     };
 
     keydown_f(e2);
+
+    const e3 = {
+        which: 37,
+        altKey: true, // let browser handle
+    };
+
+    keydown_f(e3);
 });