fix(material/stepper): switch away from animations module

Reworks the stepper so it uses CSS directly to animate, instead of going through the animations module. This both simplifies the setup and allows us to avoid the issues that come with the animations module.

src/cdk/stepper/stepper.ts

@@ -255,7 +255,7 @@ export class CdkStep implements OnChanges {
 export class CdkStepper implements AfterContentInit, AfterViewInit, OnDestroy {
   private _dir = inject(Directionality, {optional: true});
   private _changeDetectorRef = inject(ChangeDetectorRef);
-  private _elementRef = inject<ElementRef<HTMLElement>>(ElementRef);
+  protected _elementRef = inject<ElementRef<HTMLElement>>(ElementRef);
 
   /** Emits when the component is destroyed. */
   protected readonly _destroyed = new Subject<void>();

src/material/stepper/stepper-animations.ts

@@ -17,12 +17,11 @@ import {
   animateChild,
 } from '@angular/animations';
 
-export const DEFAULT_HORIZONTAL_ANIMATION_DURATION = '500ms';
-export const DEFAULT_VERTICAL_ANIMATION_DURATION = '225ms';
-
 /**
  * Animations used by the Material steppers.
  * @docs-private
+ * @deprecated No longer used, will be removed.
+ * @breaking-change 21.0.0
  */
 export const matStepperAnimations: {
   readonly horizontalStepTransition: AnimationTriggerMetadata;
@@ -43,7 +42,7 @@ export const matStepperAnimations: {
         query('@*', animateChild(), {optional: true}),
       ]),
       {
-        params: {'animationDuration': DEFAULT_HORIZONTAL_ANIMATION_DURATION},
+        params: {'animationDuration': '500ms'},
       },
     ),
   ]),
@@ -63,7 +62,7 @@ export const matStepperAnimations: {
         query('@*', animateChild(), {optional: true}),
       ]),
       {
-        params: {'animationDuration': DEFAULT_VERTICAL_ANIMATION_DURATION},
+        params: {'animationDuration': '225ms'},
       },
     ),
   ]),

src/material/stepper/stepper.html

@@ -12,52 +12,51 @@
   @case ('horizontal') {
     <div class="mat-horizontal-stepper-wrapper">
       <div class="mat-horizontal-stepper-header-container">
-        @for (step of steps; track step; let i = $index, isLast = $last) {
+        @for (step of steps; track step) {
           <ng-container
             [ngTemplateOutlet]="stepTemplate"
-            [ngTemplateOutletContext]="{step: step, i: i}"></ng-container>
-          @if (!isLast) {
+            [ngTemplateOutletContext]="{step, i: $index}"/>
+          @if (!$last) {
             <div class="mat-stepper-horizontal-line"></div>
           }
         }
       </div>
 
       <div class="mat-horizontal-content-container">
-        @for (step of steps; track step; let i = $index) {
-          <div class="mat-horizontal-stepper-content" role="tabpanel"
-               [@horizontalStepTransition]="{
-                  'value': _getAnimationDirection(i),
-                  'params': {'animationDuration': _getAnimationDuration()}
-                }"
-               (@horizontalStepTransition.done)="_animationDone.next($event)"
-               [id]="_getStepContentId(i)"
-               [attr.aria-labelledby]="_getStepLabelId(i)"
-               [class.mat-horizontal-stepper-content-inactive]="selectedIndex !== i">
-            <ng-container [ngTemplateOutlet]="step.content"></ng-container>
+        @for (step of steps; track step) {
+          <div
+            #animatedContainer
+            class="mat-horizontal-stepper-content"
+            role="tabpanel"
+            [id]="_getStepContentId($index)"
+            [attr.aria-labelledby]="_getStepLabelId($index)"
+            [class]="'mat-horizontal-stepper-content-' + _getAnimationDirection($index)"
+            [attr.inert]="selectedIndex === $index ? null : ''">
+            <ng-container [ngTemplateOutlet]="step.content"/>
           </div>
         }
       </div>
     </div>
   }
 
   @case ('vertical') {
-    @for (step of steps; track step; let i = $index, isLast = $last) {
+    @for (step of steps; track step) {
       <div class="mat-step">
         <ng-container
           [ngTemplateOutlet]="stepTemplate"
-          [ngTemplateOutletContext]="{step: step, i: i}"></ng-container>
-        <div class="mat-vertical-content-container" [class.mat-stepper-vertical-line]="!isLast">
-          <div class="mat-vertical-stepper-content" role="tabpanel"
-               [@verticalStepTransition]="{
-                  'value': _getAnimationDirection(i),
-                  'params': {'animationDuration': _getAnimationDuration()}
-                }"
-               (@verticalStepTransition.done)="_animationDone.next($event)"
-               [id]="_getStepContentId(i)"
-               [attr.aria-labelledby]="_getStepLabelId(i)"
-               [class.mat-vertical-stepper-content-inactive]="selectedIndex !== i">
+          [ngTemplateOutletContext]="{step, i: $index}"/>
+        <div
+          #animatedContainer
+          class="mat-vertical-content-container"
+          [class.mat-stepper-vertical-line]="!$last"
+          [class.mat-vertical-content-container-active]="selectedIndex === $index"
+          [attr.inert]="selectedIndex === $index ? null : ''">
+          <div class="mat-vertical-stepper-content"
+            role="tabpanel"
+            [id]="_getStepContentId($index)"
+            [attr.aria-labelledby]="_getStepLabelId($index)">
             <div class="mat-vertical-content">
-              <ng-container [ngTemplateOutlet]="step.content"></ng-container>
+              <ng-container [ngTemplateOutlet]="step.content"/>
             </div>
           </div>
         </div>
@@ -91,5 +90,5 @@
     [errorMessage]="step.errorMessage"
     [iconOverrides]="_iconOverrides"
     [disableRipple]="disableRipple || !_stepIsNavigable(i, step)"
-    [color]="step.color || color"></mat-step-header>
+    [color]="step.color || color"/>
 </ng-template>

src/material/stepper/stepper.scss

@@ -178,20 +178,30 @@
 }
 
 .mat-horizontal-stepper-content {
+  visibility: hidden;
+  overflow: hidden;
   outline: 0;
+  height: 0;
 
-  &.mat-horizontal-stepper-content-inactive {
-    height: 0;
-    overflow: hidden;
+  .mat-stepper-animations-enabled & {
+    transition: transform var(--mat-stepper-animation-duration, 0) cubic-bezier(0.35, 0, 0.25, 1);
+  }
+
+  &.mat-horizontal-stepper-content-previous {
+    transform: translate3d(-100%, 0, 0);
+  }
+
+  &.mat-horizontal-stepper-content-next {
+    transform: translate3d(100%, 0, 0);
   }
 
-  // Used to avoid an issue where when the stepper is nested inside a component that
-  // changes the `visibility` as a part of an Angular animation, the stepper's content
-  // stays hidden (see #25925). The value has to be `!important` to override the incorrect
-  // `visibility` from the animations package. This can also be solved using `visibility: visible`
-  // on `.mat-horizontal-stepper-content`, but it can allow tabbing into hidden content.
-  &:not(.mat-horizontal-stepper-content-inactive) {
-    visibility: inherit !important;
+  &.mat-horizontal-stepper-content-current {
+    // TODO(crisbeto): the height and visibility switches are a bit jarring, but that's how the
+    // animation was set up when we still used the Animations module. We should be able to make
+    // it a bit smoother.
+    visibility: visible;
+    transform: none;
+    height: auto;
   }
 }
 
@@ -209,10 +219,26 @@
 }
 
 .mat-vertical-content-container {
+  display: grid;
+  grid-template-rows: 0fr;
+  grid-template-columns: 100%;
   margin-left: stepper-variables.$vertical-stepper-content-margin;
   border: 0;
   position: relative;
 
+  .mat-stepper-animations-enabled & {
+    transition: grid-template-rows var(--mat-stepper-animation-duration, 0)
+      cubic-bezier(0.4, 0, 0.2, 1);
+  }
+
+  &.mat-vertical-content-container-active {
+    grid-template-rows: 1fr;
+  }
+
+  .mat-step:last-child & {
+    border: none;
+  }
+
   @include cdk.high-contrast {
     outline: solid 1px;
   }
@@ -221,6 +247,19 @@
     margin-left: 0;
     margin-right: stepper-variables.$vertical-stepper-content-margin;
   }
+
+
+  // All the browsers we support have support for `grid` as well, but given that these styles are
+  // load-bearing for the stepper, we have a fallback to height which doesn't animate, just in case.
+  // stylelint-disable material/no-prefixes
+  @supports not (grid-template-rows: 0fr) {
+    height: 0;
+
+    &.mat-vertical-content-container-active {
+      height: auto;
+    }
+  }
+  // stylelint-enable material/no-prefixes
 }
 
 .mat-stepper-vertical-line::before {
@@ -252,23 +291,17 @@
 .mat-vertical-stepper-content {
   overflow: hidden;
   outline: 0;
+  visibility: hidden;
 
-  // Used to avoid an issue where when the stepper is nested inside a component that
-  // changes the `visibility` as a part of an Angular animation, the stepper's content
-  // stays hidden (see #25925). The value has to be `!important` to override the incorrect
-  // `visibility` from the animations package. This can also be solved using `visibility: visible`
-  // on `.mat-vertical-stepper-content`, but it can allow tabbing into hidden content.
-  &:not(.mat-vertical-stepper-content-inactive) {
-    visibility: inherit !important;
+  .mat-stepper-animations-enabled & {
+    transition: visibility var(--mat-stepper-animation-duration, 0) linear;
+  }
+
+  .mat-vertical-content-container-active > & {
+    visibility: visible;
   }
 }
 
 .mat-vertical-content {
   padding: 0 stepper-variables.$side-gap stepper-variables.$side-gap stepper-variables.$side-gap;
 }
-
-.mat-step:last-child {
-  .mat-vertical-content-container {
-    border: none;
-  }
-}

src/material/stepper/stepper.spec.ts

@@ -34,7 +34,7 @@ import {
   inject,
   signal,
 } from '@angular/core';
-import {ComponentFixture, TestBed, fakeAsync, flush} from '@angular/core/testing';
+import {ComponentFixture, TestBed} from '@angular/core/testing';
 import {
   AbstractControl,
   AsyncValidatorFn,
@@ -364,7 +364,7 @@ describe('MatStepper', () => {
       expect(stepperComponent._getIndicatorType(0)).toBe('done');
     });
 
-    it('should emit an event when the enter animation is done', fakeAsync(() => {
+    it('should emit an event when the enter animation is done', () => {
       const stepper = fixture.debugElement.query(By.directive(MatStepper))!.componentInstance;
       const selectionChangeSpy = jasmine.createSpy('selectionChange spy');
       const animationDoneSpy = jasmine.createSpy('animationDone spy');
@@ -374,17 +374,12 @@ describe('MatStepper', () => {
       stepper.selectedIndex = 1;
       fixture.detectChanges();
 
-      expect(selectionChangeSpy).toHaveBeenCalledTimes(1);
-      expect(animationDoneSpy).not.toHaveBeenCalled();
-
-      flush();
-
       expect(selectionChangeSpy).toHaveBeenCalledTimes(1);
       expect(animationDoneSpy).toHaveBeenCalledTimes(1);
 
       selectionChangeSubscription.unsubscribe();
       animationDoneSubscription.unsubscribe();
-    }));
+    });
 
     it('should set the correct aria-posinset and aria-setsize', () => {
       const headers = Array.from<HTMLElement>(