fix(menu): account for menu padding different from the default

Currently we use the hardcoded value of the menu padding to offset the panel so the two menu triggers align. Since the value is in TS, the logic breaks down if the user has set their own padding. These changes add some logic to compute the padding the first time a sub-menu is opened.

Fixes #16167.

src/material-experimental/mdc-menu/menu.spec.ts

@@ -1713,6 +1713,26 @@ describe('MatMenu', () => {
       expect(Math.round(triggerRect.top)).toBe(Math.round(panelRect.top) + MENU_PANEL_TOP_PADDING);
     }));
 
+    it('should account for custom padding when offsetting the sub-menu', () => {
+      compileTestComponent();
+      instance.rootTriggerEl.nativeElement.style.position = 'fixed';
+      instance.rootTriggerEl.nativeElement.style.left = '75px';
+      instance.rootTriggerEl.nativeElement.style.top = '75px';
+      instance.rootTrigger.openMenu();
+      fixture.detectChanges();
+
+      // Change the padding to something different from the default.
+      (overlay.querySelector('.mat-mdc-menu-content') as HTMLElement).style.padding = '15px 0';
+
+      instance.levelOneTrigger.openMenu();
+      fixture.detectChanges();
+
+      const triggerRect = overlay.querySelector('#level-one-trigger')!.getBoundingClientRect();
+      const panelRect = overlay.querySelectorAll('.cdk-overlay-pane')[1].getBoundingClientRect();
+
+      expect(Math.round(triggerRect.top)).toBe(Math.round(panelRect.top) + 15);
+    });
+
     it('should close all of the menus when an item is clicked', fakeAsync(() => {
       compileTestComponent();
       instance.rootTriggerEl.nativeElement.click();

src/material/menu/menu-trigger.ts

@@ -58,9 +58,6 @@ export const MAT_MENU_SCROLL_STRATEGY_FACTORY_PROVIDER = {
   useFactory: MAT_MENU_SCROLL_STRATEGY_FACTORY,
 };
 
-/** Default top padding of the menu panel. */
-export const MENU_PANEL_TOP_PADDING = 8;
-
 /** Options for binding a passive event listener. */
 const passiveEventListenerOptions = normalizePassiveListenerOptions({passive: true});
 
@@ -91,6 +88,12 @@ export class MatMenuTrigger implements AfterContentInit, OnDestroy {
   private _menuCloseSubscription = Subscription.EMPTY;
   private _scrollStrategy: () => ScrollStrategy;
 
+  /**
+   * Cached value of the padding of the parent menu panel.
+   * Used to offset sub-menus to compensate for the padding.
+   */
+  private _parentInnerPadding: number | undefined;
+
   /**
    * Handles touch start events on the trigger.
    * Needs to be an arrow function so we can easily use addEventListener and removeEventListener.
@@ -450,11 +453,18 @@ export class MatMenuTrigger implements AfterContentInit, OnDestroy {
     let offsetY = 0;
 
     if (this.triggersSubmenu()) {
+      // console.log(this._parentMenu.items.first._getHostElement().offsetTop);
       // When the menu is a sub-menu, it should always align itself
       // to the edges of the trigger, instead of overlapping it.
       overlayFallbackX = originX = this.menu.xPosition === 'before' ? 'start' : 'end';
       originFallbackX = overlayX = originX === 'end' ? 'start' : 'end';
-      offsetY = overlayY === 'bottom' ? MENU_PANEL_TOP_PADDING : -MENU_PANEL_TOP_PADDING;
+
+      if (this._parentInnerPadding == null) {
+        const firstSibling = this._parentMenu.items.first;
+        this._parentInnerPadding = firstSibling ? firstSibling._getHostElement().offsetTop : 0;
+      }
+
+      offsetY = overlayY === 'bottom' ? this._parentInnerPadding : -this._parentInnerPadding;
     } else if (!this.menu.overlapTrigger) {
       originY = overlayY === 'top' ? 'bottom' : 'top';
       originFallbackY = overlayFallbackY === 'top' ? 'bottom' : 'top';

src/material/menu/menu.spec.ts

@@ -43,8 +43,9 @@ import {
   MenuPositionX,
   MenuPositionY,
 } from './index';
-import {MAT_MENU_SCROLL_STRATEGY, MENU_PANEL_TOP_PADDING} from './menu-trigger';
+import {MAT_MENU_SCROLL_STRATEGY} from './menu-trigger';
 
+const MENU_PANEL_TOP_PADDING = 8;
 
 describe('MatMenu', () => {
   let overlayContainer: OverlayContainer;
@@ -1722,6 +1723,26 @@ describe('MatMenu', () => {
       expect(Math.round(triggerRect.top)).toBe(Math.round(panelRect.top) + MENU_PANEL_TOP_PADDING);
     }));
 
+    it('should account for custom padding when offsetting the sub-menu', () => {
+      compileTestComponent();
+      instance.rootTriggerEl.nativeElement.style.position = 'fixed';
+      instance.rootTriggerEl.nativeElement.style.left = '75px';
+      instance.rootTriggerEl.nativeElement.style.top = '75px';
+      instance.rootTrigger.openMenu();
+      fixture.detectChanges();
+
+      // Change the padding to something different from the default.
+      (overlay.querySelector('.mat-menu-content') as HTMLElement).style.padding = '15px 0';
+
+      instance.levelOneTrigger.openMenu();
+      fixture.detectChanges();
+
+      const triggerRect = overlay.querySelector('#level-one-trigger')!.getBoundingClientRect();
+      const panelRect = overlay.querySelectorAll('.cdk-overlay-pane')[1].getBoundingClientRect();
+
+      expect(Math.round(triggerRect.top)).toBe(Math.round(panelRect.top) + 15);
+    });
+
     it('should close all of the menus when an item is clicked', fakeAsync(() => {
       compileTestComponent();
       instance.rootTriggerEl.nativeElement.click();