fix(expansion-panel): expand animation jumping

Fixes the expansion panel jumping towards the end of the animation due to the overflow going from `visible` to `hidden`.

Author: crisbeto

src/lib/expansion/expansion-panel.html

@@ -1,6 +1,10 @@
 <ng-content select="mat-expansion-panel-header"></ng-content>
-<div [class.mat-expanded]="expanded" class="mat-expansion-panel-content"
-     [@bodyExpansion]="_getExpandedState()" [id]="id">
+<div class="mat-expansion-panel-content"
+     [@bodyExpansion]="_getExpandedState()"
+     (@bodyExpansion.done)="_bodyAnimation($event)"
+     (@bodyExpansion.start)="_bodyAnimation($event)"
+     [id]="id"
+     #body>
   <div class="mat-expansion-panel-body">
     <ng-content></ng-content>
   </div>

src/lib/expansion/expansion-panel.scss

@@ -11,12 +11,10 @@
 }
 
 .mat-expansion-panel-content {
-  .mat-expanded & {
-    overflow: visible;
-  }
+  overflow: hidden;
 
-  &, &.ng-animating {
-    overflow: hidden;
+  &.mat-expanded {
+    overflow: visible;
   }
 }
 

src/lib/expansion/expansion-panel.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {animate, state, style, transition, trigger} from '@angular/animations';
+import {animate, state, style, transition, trigger, AnimationEvent} from '@angular/animations';
 import {
   ChangeDetectionStrategy,
   ChangeDetectorRef,
@@ -142,6 +142,22 @@ export class MatExpansionPanel extends _MatExpansionPanelMixinBase
     super.ngOnDestroy();
     this._inputChanges.complete();
   }
+
+  _bodyAnimation(event: AnimationEvent) {
+    const classList = event.element.classList;
+    const cssClass = 'mat-expanded';
+    const {phaseName, toState} = event;
+
+    // Toggle the body's `overflow: hidden` class when closing starts or when expansion ends in
+    // order to prevent the cases where switching too early would cause the animation to jump.
+    // Note that we do it directly on the DOM element to avoid the slight delay that comes
+    // with doing it via change detection.
+    if (phaseName === 'done' && toState === 'expanded') {
+      classList.add(cssClass);
+    } else if (phaseName === 'start' && toState === 'collapsed') {
+      classList.remove(cssClass);
+    }
+  }
 }
 
 @Directive({

src/lib/expansion/expansion.spec.ts

@@ -1,4 +1,4 @@
-import {async, TestBed, fakeAsync, tick, ComponentFixture} from '@angular/core/testing';
+import {async, TestBed, fakeAsync, tick, ComponentFixture, flush} from '@angular/core/testing';
 import {Component, ViewChild} from '@angular/core';
 import {By} from '@angular/platform-browser';
 import {NoopAnimationsModule} from '@angular/platform-browser/animations';
@@ -21,19 +21,22 @@ describe('MatExpansionPanel', () => {
     TestBed.compileComponents();
   }));
 
-  it('should expand and collapse the panel', () => {
+  it('should expand and collapse the panel', fakeAsync(() => {
     const fixture = TestBed.createComponent(PanelWithContent);
-    const contentEl = fixture.debugElement.query(By.css('.mat-expansion-panel-content'));
-    const headerEl = fixture.debugElement.query(By.css('.mat-expansion-panel-header'));
+    const contentEl = fixture.nativeElement.querySelector('.mat-expansion-panel-content');
+    const headerEl = fixture.nativeElement.querySelector('.mat-expansion-panel-header');
     fixture.detectChanges();
-    expect(headerEl.classes['mat-expanded']).toBeFalsy();
-    expect(contentEl.classes['mat-expanded']).toBeFalsy();
+
+    expect(headerEl.classList).not.toContain('mat-expanded');
+    expect(contentEl.classList).not.toContain('mat-expanded');
 
     fixture.componentInstance.expanded = true;
     fixture.detectChanges();
-    expect(headerEl.classes['mat-expanded']).toBeTruthy();
-    expect(contentEl.classes['mat-expanded']).toBeTruthy();
-  });
+    flush();
+
+    expect(headerEl.classList).toContain('mat-expanded');
+    expect(contentEl.classList).toContain('mat-expanded');
+  }));
 
   it('emit correct events for change in panel expanded state', () => {
     const fixture = TestBed.createComponent(PanelWithContent);