fix: should not transition states when alt-clicked

In most browsers, clicking links with the Alt key has a special behavior, for example, Chrome
downloads the target resource. As with other modifier keys, the router should stop the original
navigation to avoid preventing the browser’s default behavior.

When users click a link while holding the Alt key together, the browsers behave as follows.

Windows 10:

| Browser    | Behavior                                    |
|:-----------|:--------------------------------------------|
| Chrome 84  | Download the target resource                |
| Firefox 79 | Prevent navigation and therefore do nothing |
| Edge 84    | Download the target resource                |
| IE 11      | No impact                                   |

macOS Catalina:

| Browser    | Behavior                                    |
|:-----------|:--------------------------------------------|
| Chrome 84  | Download the target resource                |
| Firefox 79 | Prevent navigation and therefore do nothing |
| Safari 13  | Download the target resource                |

src/directives/stateDirectives.ts

@@ -95,7 +95,7 @@ function clickHook(
     const button = e.which || e.button,
       target = getDef();
 
-    if (!(button > 1 || e.ctrlKey || e.metaKey || e.shiftKey || el.attr('target'))) {
+    if (!(button > 1 || e.ctrlKey || e.metaKey || e.shiftKey || e.altKey || el.attr('target'))) {
       // HACK: This is to allow ng-clicks to be processed before the transition is initiated:
       const transition = $timeout(function () {
         if (!el.attr('disabled')) {

test/stateDirectivesSpec.ts

@@ -232,6 +232,18 @@ describe('uiStateRef', function () {
         expect(obj($stateParams)).toEqual({});
       }));
 
+      it('should not transition states when alt-clicked', inject(function ($state, $stateParams, $q) {
+        expect($state.$current.name).toEqual('top');
+        expect(obj($stateParams)).toEqual({});
+
+        triggerClick(el, { altKey: true });
+        timeoutFlush();
+        $q.flush();
+
+        expect($state.current.name).toEqual('top');
+        expect(obj($stateParams)).toEqual({});
+      }));
+
       it('should not transition states when middle-clicked', inject(function ($state, $stateParams, $q) {
         expect($state.$current.name).toEqual('top');
         expect(obj($stateParams)).toEqual({});