fix(menu): incorrectly styling keyboard focus, if trigger is right clicked before opening (#13136)

Currently `mat-menu` displays focus indication only if it has been opened by keyboard. We detect how the menu was opened using a `mousedown` event. The issue comes from the fact that `mousedown` also fires for right or middle button clicks, event though a subsequent `click` won't fire, which will throw off our detection if the user opens the menu with their keyboard afterwards. These changes add a check that ensures that only left button clicks are considered as opening.

src/lib/menu/menu-trigger.ts

@@ -454,7 +454,9 @@ export class MatMenuTrigger implements AfterContentInit, OnDestroy {
   /** Handles mouse presses on the trigger. */
   _handleMousedown(event: MouseEvent): void {
     if (!isFakeMousedownFromScreenReader(event)) {
-      this._openedByMouse = true;
+      // Since right or middle button clicks won't trigger the `click` event,
+      // we shouldn't consider the menu as opened by mouse in those cases.
+      this._openedByMouse = event.button === 0;
 
       // Since clicking on the trigger won't close the menu if it opens a sub-menu,
       // we should prevent focus from moving onto it via click to avoid the

src/lib/menu/menu.spec.ts

@@ -208,7 +208,7 @@ describe('MatMenu', () => {
       fixture.detectChanges();
       const triggerEl = fixture.componentInstance.triggerEl.nativeElement;
 
-      dispatchFakeEvent(triggerEl, 'mousedown');
+      dispatchMouseEvent(triggerEl, 'mousedown');
       triggerEl.click();
       fixture.detectChanges();
       patchElementFocus(triggerEl);
@@ -222,6 +222,31 @@ describe('MatMenu', () => {
       focusMonitor.stopMonitoring(triggerEl);
     }));
 
+  it('should set proper focus origin when right clicking on trigger, before opening by keyboard',
+    fakeAsync(() => {
+      const fixture = createComponent(SimpleMenu, [], [FakeIcon]);
+      fixture.detectChanges();
+      const triggerEl = fixture.componentInstance.triggerEl.nativeElement;
+
+      patchElementFocus(triggerEl);
+      focusMonitor.monitor(triggerEl, false);
+
+      // Trigger a fake right click.
+      dispatchEvent(triggerEl, createMouseEvent('mousedown', 50, 100, 2));
+
+      // A click without a left button mousedown before it is considered a keyboard open.
+      triggerEl.click();
+      fixture.detectChanges();
+
+      fixture.componentInstance.trigger.closeMenu();
+      fixture.detectChanges();
+      tick(500);
+      fixture.detectChanges();
+
+      expect(triggerEl.classList).toContain('cdk-program-focused');
+      focusMonitor.stopMonitoring(triggerEl);
+    }));
+
   it('should close the menu when pressing ESCAPE', fakeAsync(() => {
     const fixture = createComponent(SimpleMenu, [], [FakeIcon]);
     fixture.detectChanges();
@@ -347,7 +372,7 @@ describe('MatMenu', () => {
 
     const triggerEl = fixture.componentInstance.triggerEl.nativeElement;
 
-    dispatchFakeEvent(triggerEl, 'mousedown');
+    dispatchMouseEvent(triggerEl, 'mousedown');
     triggerEl.click();
     fixture.detectChanges();
     tick(500);